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CHAPTER 4

Electrochemical studies on self-assembled monolayers

(SAMs) of some aromatic thiols and alkoxycyanobiphenyl

thiols on gold surface

This chapter deals with our studies on self-assembled monolayers of

some aromatic and alkoxycyanobiphenyl thiols on gold surface. We have

studied and characterized the monolayer formation of 2-naphthalenethiol,

thiophenol (TP), o-aminothiophenol (o-ATP), p-aminothiophenol (p-ATP) and

alkoxycyanobiphenyl thiols with different alkyl chain lengths on Au surface

using organic solvents as an adsorbing medium. The barrier properties and

structural arrangements of these monolayers were evaluated and characterized

using electrochemical, spectroscopic and microscopic techniques. Cyclic

voltammetry and electrochemical impedance spectroscopy were extensively

used for the study of electron transfer reactions on these SAM modified surface.

It is found that the self-assembled monolayers of aromatic thiols on Au surface

generally exhibit poor electrochemical blocking ability towards the redox

reactions. In this work, we have tried to improve the blocking ability of these

aromatic SAMs on Au surface using a variety of methods such as potential

cycling, annealing and mixed SAM formation. We have used potassium

ferro/ferri cyanide {[Fe(CN)6]3-|4-}, hexaammineruthenium(III) chloride

{[Ru(NH3)6]2+|3+} complexes as redox probe molecules to evaluate the barrier

property of these aromatic monolayers on Au surface. We have studied and

characterized the monolayers of these thiol molecules on Au surface using

organic solvents as an adsorbing medium in order to compare their

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electrochemical blocking ability and structural arrangement with the

corresponding monolayers prepared using the hexagonal liquid crystalline

phase as an adsorbing medium that has been discussed in the next chapter.

This chapter has been divided into three parts. The first part deals with

the monolayer formation and characterization of 2-naphthalenethiol on Au

surface that has been studied and reported for the first time in literature. The

second part contains the study of SAM formation of TP, o-ATP and p-ATP on

polycrystalline Au surface. The effects of pH and mixed SAM formation on the

blocking ability of these monolayers have been investigated and discussed. The

third and final part deals with the monolayer formation and characterization of

alkoxycyanobiphenyl thiols with different alkyl chain lengths on Au surface.

The important feature of these thiol molecules is that it contains both the

aliphatic and aromatic groups in its structure.

I. SAM OF 2-NAPHTHALENETHIOL ON GOLD SURFACE

4.1. Introduction

Self-assembled monolayer (SAM) refers to a single layer of molecules

on a substrate, which are adsorbed spontaneously by chemisorption and exhibit

a high degree of orientation, molecular ordering and packing density [1,2].

They have received a great deal of attention in recent times due to their

potential application in a variety of fields such as sensors [3-6],

photolithography [7-9], nonlinear optical materials [10], high-density memory

storage devices [11] and corrosion protection [12,13]. Even though adsorption

of several functional groups on various metal and non-metallic surfaces have

been reported in literature, it is the thiols and disulfides on gold that have been

extensively studied in recent times [14]. The various aspects of monolayer

formation, structure, packing, orientation, wetting properties and stability have

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been examined using different techniques like Fourier transform infrared

spectroscopy (FTIR), contact angle measurements, X-ray photoelectron

spectroscopy (XPS), ellipsometry and scanning probe microscopy (SPM).

Although a complete characterization of SAMs depends on the above-

mentioned techniques, the electrochemical techniques namely cyclic

voltammetry (CV) and electrochemical impedance spectroscopy (EIS) can be

used effectively to understand the packing density, surface coverage and

distribution of pinholes and defects in the formation of the monolayer [14].

We find from literature that there are much less reports on aromatic

thiol monolayers in contrast to that of long chain aliphatic thiols. Aromatic thiol

monolayers represent interesting systems due to the highly delocalized π-

electron density and structural rigidity of the phenyl ring. There have been some

studies on thiophenol and functionalized thiophenol SAMs on gold [15-19].

Vijayamohanan et al. [20-25] characterized the monolayer formation of

naphthalene disulfide and Naphtho[1,8-cd]-1,2-dithiol on Au and Ag using

electrochemical techniques, STM, XPS and surface enhanced Raman scattering

(SERS). Apart from naphthalene disulphide, 1,4-dimethoxynaphthalene

norbornylogous compounds were also reported and it was found that the

electron transfer rate is enhanced compared to their alkane analogues due to the

rigidity of norbornylogous bridges [26]. Chang et al. reported (6-alkoxynaphth-

2-yl)methanethiol SAM on Au and Ag by characterizing them using contact

angle measurements, ellipsometry and reflection absorption IR spectroscopy

and found that the naphthalene ring forms a herringbone structure with a tilted

configuration [27]. Whitesides et al. investigated the metal-insulator-metal

junction based on SAM of 2-naphthalenethiol on Au and Ag by measuring the

breakdown voltage and found that the SAM on Au junction is mechanically

stable but immediately breaks down on application of potential [28]. To the best

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of our knowledge, there is no report in literature on the monolayer formation

and characterization of 2-naphthalenethiol on gold except by Randall and

Joseph [29]. These authors reported the formation of SAM of 2-

naphthalenethiol and bis(2-naphthyl) disulfide onto bulk gold using liquid

chromatography and diode array spectroscopy and showed that the naphthalene

ring is tilted to the Au surface normal. There is, however, no report on the

electrochemical characterization, determination of surface coverage based on

pinhole defect analysis, study on kinetics of SAM formation and structural

arrangement of SAM of 2-naphthalenethiol on Au.

In this chapter, we discuss the results of our study on the self-

assembled monolayer of 2-naphthalenethiol on Au and its characterization

using electrochemical, spectroscopic (FTIR) and microscopic (STM)

techniques. We have used electrochemical techniques such as cyclic

voltammetry (CV) and electrochemical impedance spectroscopy (EIS) to

evaluate the barrier properties of the monolayer based on its structural ordering

and packing. We have also studied the electron transfer process of two

important redox systems viz., [Fe(CN)6]3-|4- and [Ru(NH3)6]2+|3+ on the 2-

napthalenethiol monolayer modified Au electrode. In addition, EIS

measurement is used for the determination of surface coverage of the

monolayer and pinholes and defects analysis. The kinetics of adsorption of the

monolayer on Au is also investigated using impedance measurements. Grazing

angle FTIR spectroscopy, STM and interfacial capacitance studies are used to

determine the orientation and structural arrangement of SAM on Au surface.

The effects of potential cycling and annealing processes on the barrier property

and structural arrangement of the monolayer have also been investigated.

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4.2. Experimental section

4.2.1. Chemicals

All the chemical reagents used in this study were analar (AR) grade

reagents. 2-naphthalenethiol (Aldrich), ethanol 99.95% (Merck), lithium

perchlorate (Acros Organics), potassium ferrocyanide (Loba), potassium

ferricyanide (Qualigens), sodium fluoride (Qualigens), perchloric acid

(Qualigens) and hexaammineruthenium(III) chloride (Alfa Aesar) were used in

this study as received. Millipore water having a resistivity of 18 MΩ cm was

used to prepare all the aqueous solutions.

4.2.2. Electrodes and cells

Evaporated gold (~100 nm thickness) on glass with chromium

underlayers (2-5nm thickness) was used as the substrate for SAM formation and

its characterization. The substrate was heated to 3500C during gold evaporation

under a vacuum pressure of 2 X 10-5 mbar, a process that normally yields a

substrate with predominantly Au (111) orientation. The evaporated gold

samples were used as strips for electrochemical, FTIR and STM studies. We

have also carried out the electrochemical experiments with a gold wire as a

working electrode, which was constructed by a proper sealing of 99.99% pure

gold wire (Arora Mathey) of 0.5mm diameter with soda lime glass having

thermal expansion coefficient close to that of gold. The electrode has a

geometric area of 0.002 cm2. This small area-working electrode has a highly

reproducible true surface area (as measured by potential cycling in 0.1M

perchloric acid) even after repeated usage. A conventional three-electrode

electrochemical cell was used for the studies involving electrochemical

techniques. A platinum foil of large surface area was used as the counter

electrode and a saturated calomel electrode (SCE) as the reference electrode.

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The cell was cleaned thoroughly before each experiment and kept in a hot air

oven at 1000C for at least 1 hour before the start of the experiment.

4.2.3. Electrode pre-treatment and thiol adsorption

Immediately before use, the gold wire electrode was polished with

emery papers of grade 800 and 1500 respectively followed by polishing in

aqueous slurries of progressively finer alumina (1.0, 0.3 and 0.05 μm sizes) and

then ultrasonicated to remove alumina particles. Then it was cleaned in a

“piranha” solution (a mixture of 30% H2O2 and Conc.H2SO4 in 1:3 ratio;

Caution! Piranha solution is very reactive with organic compounds, storing in

a closed container and exposure to direct contact should be avoided). Finally it

was rinsed in distilled water thoroughly followed by rinsing in millipore water

before SAM formation. In the case of evaporated Au, the strips were used for

SAM formation and it was pretreated with “piranha” solution. The monolayers

were prepared by keeping the gold electrode in 20mM 2-naphthalenethiol in

ethanol for about 1hour. After the adsorption of thiol, the electrode was rinsed

with ethanol, distilled water and finally with millipore water. For the grazing

angle FTIR spectroscopy and STM analysis, the evaporated Au samples were

used as strips for thiol adsorption.

4.2.4. Electrochemical characterization

Cyclic voltammetry and electrochemical impedance spectroscopy were

used for the electrochemical characterization of SAM-modified electrodes. The

barrier property of the monolayer has been evaluated by studying the electron

transfer reaction using two different redox probes namely potassium

ferrocyanide (negative redox probe) and hexaammineruthenium(III) chloride

(positive redox probe). Cyclic voltammetry was performed in two different

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solutions viz., 10mM potassium ferrocyanide with 1M sodium fluoride as a

supporting electrolyte at a potential range of –0.1V to 0.5V vs. SCE and 1mM

hexaammineruthenium(III) chloride with 0.1M lithium perchlorate as the

supporting electrolyte at a potential range of –0.4V to 0.1V vs. SCE. We have

used a potential sweep rate of 50 mV/s for all the cyclic voltammetric

measurements. The impedance measurements were carried out using an ac of

10mV amplitude signal at a formal potential of the redox couple, in solution

containing always equal concentrations of both the oxidized and reduced forms

of the redox couple namely, 10mM potassium ferrocyanide and 10mM

potassium ferricyanide in 1M NaF. A wide frequency range of 100kHz to 0.1Hz

was used for the impedance studies. The gold oxide stripping analysis was

conducted in 0.1M perchloric acid in the potential range of 0.2V to 1.4V vs.

SCE. Adsorption kinetics studies of monolayer formation were carried out in

10μM 2-naphthalenethiol + 0.1M LiClO4 in ethanol solution at different

potentials using the evaporated Au electrodes as strips by measuring the real

and imaginary components of impedance. All the experiments were performed

at room temperature.

4.2.5. Instrumentation

Cyclic voltammetry was carried out using an EG&G potentiostat

(model 263A) interfaced to a personal computer (PC) through a GPIB card

(National Instruments). The potential ranges and scan rates used for the studies

are shown in the respective diagrams. For electrochemical impedance

spectroscopic studies, the potentiostat was used along with an EG&G 5210 lock

in amplifier controlled by Power Sine software. For adsorption kinetics studies

of monolayer formation an EG&G potentiostat (model 263A) along with an

EG&G 5210 lock in amplifier interfaced to a PC was used. The real and

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imaginary components of impedance were acquired at a sampling rate of 100ms

for initial 20s and at 1s for the remaining time of monolayer adsorption. The

data acquisition was carried out using PCL 203A (Dyanalog) data acquisition

card interfaced with LabTech notebook software.

A home built scanning tunneling microscope (STM) in high-resolution

mode described elsewhere [30] was used to probe the SAM modified gold

surface. STM images were obtained at room temperature in air and the

instrument was operated in constant current mode with a tunneling current of

1nA at a sample bias voltage of +100mV. Prior to these experiments, the

instrument was calibrated with highly oriented pyrolytic graphite (HOPG)

(ZYA grade, Advanced Ceramic). An electrochemically etched tungsten tip was

used as the probe. The images shown here were plane corrected and Fourier

filtered using scanning probe image processor (SPIP) software (Image

Metrology, Denmark). To ensure that the images shown were representative of

the monolayer morphology, multiple images were taken at different locations

and scan ranges.

The FTIR spectra were obtained using a FTIR 8400 model

(SHIMADZU) with a fixed 850 grazing angle attachment (FT-85; Thermo

Spectra-Tech).

4.3. Results and discussion

4.3.1. Electrochemical characterization

Cyclic voltammetry and electrochemical impedance spectroscopy have

been used for the electrochemical characterization of the monolayer of 2-

naphthalenethiol on Au surface. Figure 1 shows the cyclic voltammograms of

bare gold electrode (Fig. 1(a)) and 2-naphthalenethiol modified gold electrode

(Fig. 1(b)) in 10mM potassium ferrocyanide with 1M NaF as the supporting

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electrolyte at a potential scan rate of 50 mV/s. It can be seen that the bare gold

electrode shows a reversible peak for the redox couple indicating that the

reaction is diffusion controlled. In contrast, for the SAM modified electrode, the

CV exhibits an irreversible behaviour with a large peak separation and

significant peak currents indicating that the electron transfer reaction occurs by

an irreversible process. Such a behaviour is typical of a monolayer with rather

poor blocking properties to the redox species.

Figure 2 shows the impedance plots of the bare gold electrode and

SAM modified electrode in equal concentrations of potassium ferro/ferri

cyanide with 1M NaF as the supporting electrolyte. Figure 2 (a) shows a

straight line at low frequency and a very small semicircle at high frequency

region indicating that the process is essentially diffusion-controlled for the

redox couple on a bare gold electrode. The impedance plot of SAM modified

electrode in Figure 2 (b) shows the behaviour of a blocking monolayer with a

well-defined semicircle at higher frequencies.

Figure 1: Cyclic voltammograms of (a) bare Au electrode and (b) SAM of 2-

naphtalenethiol modified Au electrode in 10mM potassium ferrocyanide with

1M NaF as a supporting electrolyte at a potential scan rate of 50 mV/s.

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These impedance values are fitted to a standard Randle’s equivalent

circuit model having parallel Rct and CPE (constant phase element) components

with a series uncompensated solution resistance Ru. From the fitting, a charge

transfer resistance (Rct) of 0.55 Ω cm2 is measured for the bare gold electrode

while the monolayer-modified electrode shows a higher Rct value of 190 Ω cm2.

This value is lower compared to the Rct value of 568 Ω cm2 reported for the

SAM of naphthalene disulphide on Au by Vijayamohanan et al. [25].

Figure 2: Impedance plots in 10mM potassium ferrocyanide and 10mM

potassium ferricyanide with 1M NaF as the supporting electrolyte for (a) bare

Au electrode and (b) monolayer of 2-naphthalenethiol coated Au electrode.

We have tried to improve the blocking behaviour of the monolayer-

modified electrode by two different processes, namely, by subjecting the freshly

formed SAM to (i) potential cycling in a pure supporting electrolyte and (ii)

annealing at 720±20C.

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4.3.2.Effect of potential cycling and annealing

Immediately after the monolayer formation, the electrode is potential

cycled between 0.0 V to 0.6 V vs. SCE in 1M NaF aqueous solution without

any redox species and the same electrode is used for the cyclic voltammetry and

impedance spectroscopic studies to determine the barrier property of the

monolayer. In the case of annealing, after the thiol adsorption, the gold

electrode is taken out, washed completely and immediately kept in a hot air

oven for about half an hour at 720±20C. It was then allowed to cool down to

room temperature and used for the analysis.

Figure 3 shows, respectively, the CVs of the monolayer-modified

electrode, potential cycled and annealed (after the SAM formation) electrodes

in 10mM potassium ferrocyanide with 1M NaF as the supporting electrolyte

and at a potential scan rate of 50 mV/s. For comparison, the cyclic

voltammogram of bare gold electrode for the same redox reaction is also shown

in the inset of Figure 3. Figure 3(a) shows an irreversible peak formation for the

SAM modified electrode. In contrast, the potential cycled electrode shows a

better blocking behaviour for the redox reaction (Fig. 3(b)). However the barrier

property of the SAM modified electrode has dramatically improved after

annealing as can be seen from the figure 3(c). It is evident form the CVs that the

electron transfer reaction through the monolayer film is almost totally inhibited,

a process that can only be attributed to a more compact organization and

structural modification of the monolayer after annealing.

Figures 4(a) and (b) show respectively, the impedance plots of

potential cycled and annealed electrodes (after SAM formation), in equal

concentrations of potassium ferro/ferri cyanide with NaF as the supporting

electrolyte. The impedance values are fitted to standard Randle’s equivalent

circuit comprising of faradaic impedance Zf, which is parallel to a constant

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phase element (CPE) represented by Q and in series with the uncompensated

solution resistance Ru. The faradaic impedance Zf is a series combination of

charge transfer resistance Rct and the Warburg impedance W.

Table 1 shows the values obtained from the equivalent circuit fitting of

impedance values for various samples. It can be seen from the table that the Rct

values, as expected are higher for the monolayer-modified electrodes when

compared to that of bare gold electrode. The SAM modified electrode after

potential cycling in 1M NaF shows a very little increase in the Rct value when

compared to the fresh monolayer coated electrode, which implies that the

charge transfer process is inhibited only marginally by the potential cycling

process. This is in contrast to the CV behaviour as seen from figure 3(b) for the

electrode, which is potential cycled, the shape of which suggests a more

blocking behaviour than the freshly formed SAM. This behaviour implies a

possibility of some structural re-organization of the monolayer due to potential

cycling without significantly affecting the ionic permeability to the redox

species, as revealed by the Rct values. Such behaviour was also reported earlier

in the case of aliphatic thiol monolayers [31-34]. The fact that the Rct value is

very high (3854 Ω cm2) for the annealed SAM suggests that the barrier property

of the monolayer to the electron transfer reaction has improved remarkably and

it is almost completely under charge transfer control. The impedance diagram

of the redox reaction on the annealed SAM (Fig. 4(b)) shows a large semicircle

in the entire range of frequencies. This is fitted to a parallel combination of Q

(CPE) and charge transfer resistance, Rct with a series uncompensated solution

resistance Ru. The Rct value of 3854 Ω cm2 reported here for the SAM of 2-

naphthalenethiol on Au is very much higher when compared to the literature

values of 568 Ω cm2, 1100 Ω cm2 and 1300 Ω cm2 for the respective SAMs of

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naphthalene disulphide (NDS), diphenyldiselenide (DDSe) and

diphenyldisulphide (DDS) on gold reported by Vijayamohanan et al. [22,24,25],

which implies the compact and highly dense formation of SAM of 2-

naphthalenethiol on Au surface.

Figure 3: Cyclic voltammograms in 10mM potassium ferrocyanide with 1M

NaF as supporting electrolyte at a potential scan rate of 50 mV/s for, (a) SAM

of 2-naphthalenethiol modified Au electrode, (b) potential cycled electrode after

the SAM formation of 2-naphthalenethiol on Au, and (c) annealed electrode

after the SAM formation of 2-naphthalenethiol on Au surface. Inset shows the

cyclic voltammogram of bare Au electrode in the same solution for comparison.

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Table-1

Rct values obtained from the equivalent circuit fitting analysis of impedance

data for various electrodes.

Figure 4: Impedance plots in an aqueous solution of 10mM potassium

ferrocyanide and 10mM potassium ferricyanide with 1M NaF as the supporting

electrolyte for, (a) potential cycled electrode after the monolayer formation,

and (b) annealed electrode after the monolayer formation.

Sample

Rct (Ω cm2)

[Fe(CN)6]3-|4-

Rct (Ω cm2)

[Ru(NH3)6]2+|3+

Bare Au 0.55 1.26

SAM of 2-NT on Au 190 5.00

Potential cycled SAM 221 11.59

Annealed SAM 3854 34.75

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From the impedance diagram (Fig. 4), it can be seen that there is a low

frequency component in the case of the SAM modified electrodes (including for

the potential cycled and annealed samples), which can arise due to slow ionic

diffusion through the pinholes and defects present in the SAM. This low

frequency diffusion region is significantly less in the case of annealed sample.

For the equivalent circuit fitting analysis of impedance values, we have

considered only the semicircle component and fitted the data from 100 KHz to

1Hz. The measured Rct values are shown in the Table 1.

We have also used hexaammineruthenium(III) chloride as a redox

probe to determine the barrier property of the monolayer of 2-naphthalenethiol

on Au surface. Figures 5(a-c) show the CVs obtained for the bare gold

electrode, SAM coated and annealed electrodes respectively. It can be seen

from the figure that the ruthenium redox reaction is not blocked even in the case

of annealed SAM electrode, which is very much in contrast to the behaviour of

the potassium ferrocyanide redox system. It can also be seen that the

monolayer-coated electrode shows nearly reversible peaks for the ruthenium

redox reaction showing facile nature of the electron transfer process with the

reaction under diffusion control.

Figure 6 shows the impedance plots of the bare gold and SAM

modified electrodes (including the potential cycled and annealed electrodes) in

1mM hexaammineruthenium(III) chloride with 0.1M LiClO4 as the supporting

electrolyte at the formal potential. The equivalent circuit fitting analysis of the

impedance plots shows that there is a small increase in the Rct values for the

SAM modified electrodes when compared to the bare gold electrode. The high

frequency part of the impedance data shows a small semicircle as can be seen

from the inset of the respective diagrams. The impedance analysis also indicates

that there is a small increase in Rct value with the potential cycling and

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annealing processes. This implies that the electron transfer process is not

significantly inhibited in the case of ruthenium redox reaction. Since the

electron transfer reaction is almost fully inhibited in the case of ferrocyanide

ions, which has a comparable ionic size (0.64nm for the ruthenium hexamine

complex vs. 0.60nm for the potassium ferrocyanide complex [19]), it is difficult

to conclude that ruthenium ions can have access to the electrode surface

through the pinholes and defects present in the monolayer and ferrocyanide

cannot.

Figure 5: Cyclic voltammograms in 1mM hexaammineruthenium(III) chloride

with 0.1M LiClO4 as supporting electrolyte at a potential scan rate of 50 mV/s

for, (a) bare Au electrode, (b) SAM of 2-naphthalenethiol modified Au electrode

and (c) annealed electrode after the SAM formation.

210

Figure 6: Impedance plots in 1mM hexaammineruthenium(III) chloride with

0.1M LiClO4 as the supporting electrolyte for, (a) bare Au electrode, (b), SAM

of 2-naphthalenethiol coated Au electrode, (c) potential cycled electrode after

the monolayer formation and (d) annealed electrode after the monolayer

formation. Inset shows the zoomed impedance plots of the above-mentioned

electrodes at high frequency region, indicating the marginal increase in the Rct

values of the respective electrodes.

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In general, the electron transfer reaction on the SAM modified gold

electrodes can occur either through pinholes and defects present in the

monolayer or by tunneling process. In the case of aromatic thiols the electron

transfer can be further facilitated by the delocalized π-electrons present in the

molecule due to extended conjugation. It is well known that the long chain

aliphatic thiols form a well-packed, highly dense monolayer, free of measurable

pinholes and act as a barrier to the electron transfer reaction and ion

penetration. If the electron transfer occurs through the pinholes and defects

present in the monolayer, then the response of cyclic voltammogram will

depend upon the distribution of pinholes and defects present in the structure of

the monolayer. A linear diffusion process is expected when the pinholes are

close enough for the diffusion layers to be overlapping with each other

facilitating the electron transfer reaction. If the pinholes and defects are more

widely spaced and evenly distributed, then there is no overlap of the diffusion

layer and the pores constitute an array of microelectrodes leading to a radial

diffusion. In the case of SAM of 2-naphthalenethiol on Au, the response

obtained in the cyclic voltammogram of [Fe(CN)6]3-|4- redox couple (Fig. 1(b))

shows that the linear diffusion is dominant and the electron transfer occurs

through the gaps available in the monolayer. The potential cycling effect leads

to an effective packing of the monolayer with much less pinholes and defects

(Fig. 3(b)). After annealing, the radial diffusion process dominates and the

electrode behaves like a microelectrode array with an excellent blocking of the

redox reaction (Fig. 3(c)).

We have carried out several of experiments intended to evaluate the

integrity of the monolayer and to rule out the possibility of any reaction product

blocking the [Fe(CN)6]3-|4- redox reaction. One of the experiments involved first

carrying out CV studies in the potential range from –0.4V to 0.1V vs. SCE

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corresponding to that of [Ru(NH3)6]2+|3+ redox reaction where a reversible CV is

obtained (Fig. 7(a)). This is followed by scanning the same electrode in the

potential region from –0.1V to 0.5V vs. SCE corresponding to [Fe(CN)6]3-|4-

redox reaction. Here the CV (Fig. 7(b)) shows a fully blocking behaviour.

Further scanning at the potential range of [Ru(NH3)6]2+|3+ redox reaction again

exhibits a reversible CV (Fig. 7(c)). From these experiments we show that the

monolayer is neither desorbed nor undergo any structural transformation within

these potential ranges. It is very clear from the above series of experiments that

the monolayer of 2-naphthalenethiol on Au is selective to ruthenium redox

reaction and blocks potassium ferrocyanide reaction. These studies also prove

that 2-naphthalenethiol does not undergo any redox reaction within the potential

range of our study. Figure 7 shows the CV responses for the above-mentioned

series of experiments.

Figure 7: Cyclic voltammograms of SAM of 2-naphthalenethiol on Au coated

electrodes at a potential scan rate of 50mV/s in, (a) 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as the supporting

electrolyte. (b) 10mM potassium ferrocyanide with 1M NaF as the supporting

electrolyte after the above scan. (c) Again in 1mM hexaammineruthenium(III)

chloride with 0.1M LiClO4 as the supporting electrolyte after the above scan.

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We have also studied the potential dependence of interfacial

capacitance of the monolayer of 2-naphthalenethiol on Au surface. Figure 8

shows a plot of change of interfacial capacitance of the monolayer-coated

electrode with the potential. These studies clearly indicate that the monolayer is

stable and intact on the electrode surface at the potential ranges used for the

study of electron transfer processes of both potassium ferro/ferri cyanide and

hexaammineruthenium(III) chloride redox couples. The measured capacitance

values in 1M NaF aqueous solution also do not show any large changes within

in the potential range of -0.6V to +0.8V vs. SCE except at extreme ranges used

for the analysis. This means that the 2-naphthalenethiol does not desorb or

undergo any redox reaction on the electrode surface at this potential range. It is

also clear from these studies that the potential dependent structural change of

monolayer has not occurred at the potential range of the [Ru(NH3)6]2+|3+ redox

reaction which would have been reflected in abnormal increase in the

capacitance value. This also rules out the possibility of

hexaammineruthenium(III) species penetrating through the film and undergoing

the redox reaction.

Figure 8: A plot of change of interfacial capacitance of SAM of 2-

naphthalenethiol on Au coated electrode with potential.

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The fact that the annealed monolayer completely blocks the electron

transfer reaction in the case of ferrocyanide system and allows the ruthenium

redox reaction shows that the SAM, far from acting as an effective barrier,

actually facilitates the electron transfer reaction of ruthenium complex. It is

well known that the hexaammineruthenium(III) chloride undergoes outer sphere

electron transfer reaction in which the redox species approaches the SAM

modified electrode surface and exchanges electron with the metal at a minimal

distance with the hydration shell remaining intact [35]. This is in contrast to that

of redox reaction of potassium ferro/ferri cyanide system, which involves the

formation of reaction intermediates getting adsorbed on the electrode surface. In

this case, the electron transfer is accompanied by a change in the structure of

the coordination shell thereby involving bond rearrangements [36]. We feel that

such a process is inhibited due to inaccessibility of the electrode surface to the

redox species. The electron transfer process in the case of

hexaammineruthenium(III) chloride reaction can take place by tunneling

provided the process is facilitated by a suitable mediator. The tunneling process

via the alkyl chain is well known in the case of alkanethiols [37-40] and

oligophenylenevinylene bridges between a gold electrode and a tethered redox

species in contact with an aqueous electrolyte [41]. However the electron

transfer by tunneling will exhibit distance dependence and when the redox

species is separated by several Å from the surface, the process should slow

down and follow charge transfer kinetics. For a distant dependent electron

transfer for a donor-bridge-acceptor system, the current (I) is exponentially

related to the distance separating the centers of donor-acceptor (R) by the

relationship I ∝ e -βR where β is the inverse decay length which is a structure

related attenuation factor for the SAM. While this factor is around 0.8Å–1 for

alkanethiols [40], it is generally low (0.1-0.6Å–1) for oligophenylene thiols [42]

215

and other conjugated oligomers [43], which is attributed to delocalization of

donor and acceptor states on the bridge.

The fact that the reaction is not inhibited to a significant extent with

diffusion control at lower frequencies points to a tunneling process where the π-

electrons in the naphthalene ring facilitates the electron transfer between the

ruthenium complex and the gold electrode. It has been earlier reported that in

the case of both aromatic and hetero aromatic thiols, the electron transfer

reaction is facilitated by the presence of highly delocalized π-electrons in the

aromatic ring and also by the extended conjugation of the molecules [44-46].

This property of the aromatic SAM is extensively used for sensor and catalysis

applications [47-49]. The aromatic SAM can also be used as a molecular bridge

to study the effect of different interaction forces, ionic and hydrogen bonding in

the long-range electron transfer reaction. The observed results were attributed to

the tunneling process due to the presence of delocalized π-electrons in the

aromatic ring, which acts as a bridge between the electrode surface and the

redox probe [50-52]. In our case, the facile nature of electron transfer reaction

of the ruthenium complex at the SAM-solution interface suggests a tunneling

process, where the naphthalene ring with the delocalized π-electrons acts as a

bridge between the monolayer-modified electrode and ruthenium complex.

4.3.3. Study of kinetics of adsorption of 2-naphthalenethiol on Au

While the kinetics of adsorption of alkanethiol SAMs on gold has been

well studied [53-57], there are very few reports in literature on the kinetics of

SAM formation of aromatic thiols. A study of the adsorption kinetics provides

information on the monolayer evolution and structural organization with time.

In the case of alkanethiol, the kinetics of monolayer formation is fairly clear.

216

The adsorption process generally follows the Langmuir kinetics at higher

concentrations and diffusion controlled Langmuir model at lower

concentrations of aliphatic thiols [53-57].

In the present work, we have studied the adsorption kinetics of

2-naphthalenethiol on gold using electrochemical impedance spectroscopy. In

electrochemical systems, one of the earliest methods used for the study of

adsorption kinetics is the measurement of interfacial capacitance [58]. It is well

known that the capacitance of the electrical double layer precisely describes the

adsorption properties and is being used widely in the study of self-assembled

monolayer [59]. The monolayer forms a dielectric barrier between the metal

and the electrolyte. The interfacial capacitance of a Metal-SAM-electrolyte

interface consists of a series combination of the capacitance of SAM and

capacitance of the diffuse layer. The overall capacitance is dominated by the

smaller of the two, namely, the capacitance of SAM [1,2].

The adsorption kinetics of 2-naphthalenethiol monolayer on Au has

been followed by measuring the impedance as a function of time using 10μM

solution of 2-naphthalenethiol and 0.1M LiClO4 in ethanol. The kinetics of

SAM formation was carried out at two different potentials viz., 0V and 0.175V

vs. SCE. From the imaginary component of impedance at the high frequency

pseudo plateau region, the capacitance of the monolayer was measured as a

function of time [56,57]. The surface coverage (θ) was obtained using the

formula,

θ = (C0 – Ci) / (C0 – Cf) (1)

where C0 is the bare gold capacitance, Ci is the capacitance at any time t and Cf

is the capacitance of the fully covered monolayer. Figures 9(a) and (b) show the

plots of surface coverage with time for two different potential values. There are

217

two distinct time constants that can be seen from the θ-t plots. During the first

fast step (from 0s to about 150s) almost 80-85% of coverage is completed and

the second slower step (150s-1000s) extends to the almost complete coverage of

the monolayer. We have fitted the data up to 1000s. However it can be seen

from the figures that the complete coverage occurs beyond 3000s. We have

observed some fluctuations in the surface coverage values beyond 2000s as can

be seen from the Fig. 9. However such a behaviour had not been reported in the

case of adsorption of alkanethiols on Au [53-57]. These fluctuations signify the

rearrangement of molecules on the surface during the monolayer formation. We

have calculated rate constants for the adsorption kinetics based on Langmuir

and Diffusion controlled Langmuir (DCL) adsorption isotherms, which are

represented as follows,

θ(t) = [1- exp (- km t)] (2)

θ(t) = [1- exp (-km√t)] (3)

where θ(t) is the surface coverage at any instant time t and km is the rate

constant of adsorption.

The insets of figures 9(a) and (b) show the plots of (1-θ) vs. time for

the duration of the first time constant for the respective potentials. We obtained

an excellent fit for these plots using Langmuir adsorption isotherm. We find

that the rate constant values obtained from the first time constant, km for 2-

naphthalenethiol adsorption varies with the applied potential viz., 0.0588s-1 and

0.0379s-1 for 0V and 0.175V vs. SCE respectively. The initial rate of adsorption

is distinctly faster at 0V vs. SCE. However the full coverage is reached only

beyond 2500s. In the case of 0.175V vs. SCE though the initial rate of

adsorption is slower, the full coverage is reached at the end of 1000s. We have

used concentration independent rate constant ka in order to compare with the

218

rate of adsorption of alkanethiols as reported in literature [53-57]. We find that

ka value obtained for 2-naphthalenethiol at 0.175V vs. SCE is close to that of

alkanethiol value of about 2800 M-1s-1 [56]. However the rate of adsorption ka

of 5880 M-1s-1 at 0V vs. SCE is very much higher.

Figure 9: Surface coverage (θ) vs. time plots for 10μM 2-naphthalenethiol +

0.1M LiClO4 in ethanol at potentials of (a) 0.175V and (b) 0V vs. SCE. The

insets show the respective plots of (1-θ) vs. time during the initial duration of

the adsorption corresponding to the first time constant.

The interfacial capacitance value of SAM modified electrode was

determined to be 2.76 ± 0.13μF/cm2, which is somewhat higher than the

theoretical value of the capacitance for the SAM of 2-naphthalenethiol on Au

obtained using the following expression,

C = ε ε0 A / d (4)

where C is the capacitance, ε is the dielectric constant, ε0 is the permitivity of

vacuum and d is the thickness of the monolayer. From the measured

capacitance value, using the above equation we have calculated the thickness of

the monolayer to be 8.02Å. Since the molecular length of 2-naphthalenethiol is

219

9.3Å, the lower value of the monolayer thickness obtained implies that the

molecules are tilted from the surface normal. From the molecular length of

9.3Å for 2-naphthalenethiol and a monolayer thickness of 8.02Å, we have

determined the tilt of the molecule to be about 300 from the Au surface normal.

4.3.4. Pore size analysis of the 2-naphthalenethiol SAM on Au

Electrochemical impedance spectroscopic studies on the SAM

modified electrodes can provide valuable information on the distribution of

pinholes and defects in the monolayer. Finklea et al. [60] developed a model for

the impedance response of a monolayer-modified electrode, which behaves as

an array of microelectrodes. Fawcett and coworkers have extensively studied

the monolayer integrity by pore size analysis using electrochemical impedance

data [61-63]. Based on the work of Matsuda [64] and Amatore [65], a model

has been developed to fit the faradaic impedance data obtained for the electron

transfer reactions at the SAM modified electrode, to understand the distribution

of pinholes in the monolayer. The impedance expressions have been derived by

assuming that the total pinhole area fraction, (1-θ) is less than 0.1, where θ is

the surface coverage of the monolayer. Both the real and imaginary parts of the

faradaic impedance values are plotted as a function of ω-1/2. There are two

limiting cases for this theory to be applied. At higher frequencies the diffusion

profiles of each individual microelectrode constituent of the array is separated

in contrast to the situation at lower frequencies where there is an overlap.

In the case of SAM of 2-naphthalenethiol on Au, the presence of

pinholes and defects are analyzed using the above-mentioned model. Figures 10

(a-e) show the real and imaginary parts of the faradaic impedance of different

220

SAM modified systems plotted as a function of ω-1/2. For comparison the plot of

bare gold electrode is also shown in figure 10(a). Figures 10 (b) and (c) show

the plots of real component of faradaic impedance Z′f vs. ω-1/2 for the

monolayer-coated electrode and annealed SAM electrodes respectively. Figures

10(d) and (e) show the plots of imaginary component of the faradaic impedance

Z′′f vs. ω-1/2 for the respective above-mentioned electrodes. The faradaic

impedance plots show the features similar to that of an array of microelectrodes.

There are two linear domains at high and low frequencies for the Z′f vs. ω -1/2

plot and a peak formation in the Z′′f vs. ω-1/2 plot corresponding to the frequency

of transition between the two linear domains. According to the model proposed,

this frequency separates the two time dependent diffusion profiles for the

microelectrodes.

The surface coverage of the monolayer can be obtained from the slope

of the Z′f vs. ω-1/2 plot at a higher frequency region and it is given by,

m = σ + σ / (1 - θ) (5)

where m is the slope, θ the surface coverage of the monolayer and σ the

Warburg coefficient, which can be obtained from the unmodified bare gold

electrode.

From the analysis of Fig. 10(b) and (c), a surface coverage value of

99% is obtained for the monolayer-modified electrode, which increases to a

value of 99.5% after the annealing effect. It is proposed that the thermal

annealing process eliminates any trapped solvent molecules (ethanol) during the

adsorption process and helps in the formation of a highly dense, well-packed

and compact monolayer.

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Figure 10: Plots of real part of the faradaic impedance (Z′f) vs. ω-1/2 for, (a)

bare Au electrode, (b) SAM of 2-naphthalenethiol coated Au electrode, and (c)

annealed electrode after the SAM formation. Plots of imaginary part of the

faradaic impedance (Z′′f) as a function of ω-1/2 for, (d) SAM of 2-

naphthalenethiol modified Au electrode and (e) annealed electrode after the

monolayer formation.

222

4.3.5. Gold oxide stripping analysis

The cyclic voltammogram of the gold electrode in 0.1M HClO4 shows

the usual features due to gold oxide formation during the positive potential

scanning and oxide stripping upon reversing the scan direction. The quantity of

electric charge passed during its removal of oxide is proportional to the

effective electrode area. By comparing the charge of a SAM modified electrode

with the respective charge of the bare gold electrode, the surface coverage of

the monolayer can be measured qualitatively.

Figure 11 shows the cyclic voltammograms of gold oxidation in 0.1M

HClO4. Figure 11(a) shows the gold oxide formation and stripping peaks for a

bare gold electrode. Figures 11(b) and (c) show the CVs for the SAM of 2-

naphthalenethiol on Au before and after thermal annealing respectively.

Compared to bare gold electrode, the measured charge on the monolayer-

modified electrodes is significantly less as expected. It can be seen that the

charge values measured for these two electrodes before and after annealing,

from the cathodic stripping peaks are almost the same (350 nC/cm2). The area

fraction of the coverage is calculated to be 0.95 from the formula (1-QSAM / QAu)

where the QSAM is the charge measured for the monolayer-covered electrode and

QAu for the bare gold electrode. This coverage fraction is much smaller than the

value obtained from the impedance plot viz., 0.99 – 0.995. This difference in

the values can be attributed to the fact that smaller OH– ions have easier access

to the electrode surface compared to the bulkier ferrocyanide ions, which are

used as coverage probes in these two methods.

However, after 3 to 4 cycles in 0.1M HClO4, the voltammogram is

almost similar to that of the bare gold electrode indicating the complete removal

of the monolayer. The desorption of the monolayer is confirmed by the fact that

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the potassium ferrocyanide electron transfer reaction produces reversible peaks

on the electrode which was subjected to the oxide stripping experiment. This

indicates that the monolayer is not electrochemically stable when cycled to very

high positive potentials.

Figure 11: Cyclic voltammograms of gold oxide stripping analysis in 0.1M

HClO4 for, (a) bare Au electrode, (b) SAM of 2-naphthalenethiol modified Au

electrode and (c) annealed SAM after the monolayer formation.

4.3.6. Grazing angle FTIR spectroscopy

We have carried out grazing angle FTIR spectroscopy on SAM of

2-naphthalenethiol on Au to characterize its orientation. Figures 12(a) and (b)

show the FTIR spectrum of monolayer modified Au surface before and after the

annealing effect respectively. The bulk spectrum of 2-naphthalenethiol is also

shown in the figure 12(c). The multiple peaks between 1600 to 1700 cm-1 are

224

due to the aromatic ring C-C stretching mode. The peaks at 916 and 853 cm-1

are due to the out of plane bending modes of aromatic C-H group. The presence

of peaks due to aromatic ring C-C stretching mode and out of plane bending of

aromatic C-H group implies that the naphthalene ring is not lying parallel to the

Au surface. However we do not see any peak at 3050 cm-1 due to the stretching

mode of aromatic ring C-H group as it is obscured by the rapid fall of detector

sensitivity in that region.

The striking feature of the two spectra is the positive shift in the

wavenumber for the aromatic ring C-C stretching mode after annealing. There

is a positive peak shift by ~4-5 cm-1 which can be attributed to the formation of

a highly ordered and compact monolayer after annealing process. The annealing

effect improves the packing density of the monolayer by making the aromatic

ring moiety to be tilted slightly, thereby increasing the van der Waals

interaction among the aromatic rings. These results show that the effect of

annealing leads to the formation of a well packed highly oriented monolayer of

2-naphthalenethiol on Au surface.

225

Figure 12: Grazing angle FTIR spectra of (a) monolayer of 2-naphthalenethiol

coated Au electrode, (b) annealed electrode after the monolayer formation and

(c) bulk 2-naphthalenethiol in KBr.

226

4.3.7. Scanning tunneling microscopy (STM)

STM studies have been used to characterize the structure of the

monolayer at molecular level and also to understand nature of the adsorbed

film. Figure 13(a) shows the constant current STM image of bare gold surface,

which was used for forming the monolayer film at 6nm x 6nm range. Though

the atomic features could not be resolved, the surface has a predominantly Au

(111) orientation as determined by X-ray diffraction studies. Figure 13(b)

shows the Fourier filtered constant height images of 2-naphthalenethiol

monolayer adsorbed on the evaporated gold surface at 6nm x 6nm range. The

chemisorbed S-S bond distance is determined to be 5Å in the adjacent columns

of the image (Fig. 13(c)). The vertical bright spots correspond to the individual

molecules of 2-naphthalenethiol adsorbed on Au surface. These molecules,

which are arranged in diagonal rows, are separated by a distance of 8.4Å. It can

be seen that the area per molecule corresponds to 42Å2 (8.4Å x 5Å). This value

almost exactly corresponds to the observed value of 41.4Å2 per naphthalene

molecule on a smooth polycrystalline Au surface [29]. Again, from the STM

image (Fig. 13), the monolayer coverage on the gold surface is calculated to be

about 4x10–10 mol per true surface area (roughness factor of gold surface used

for STM measurements is 1.6). A closer look on the image shows that the high

electron density region in the individual molecular space spreads to about 6Å, a

rather large value for the naphthalene ring in upright orientation. This means

that the molecule is canted with respect to the Au surface normal, which is in

agreement with the earlier results of Kolega and Schlenoff [29]. This structural

orientation of naphthalene ring on Au surface obtained for the monolayer using

STM is again in conformity with our earlier results of both the electron transfer

reaction of ruthenium complex (based on CV and EIS) and grazing angle FTIR

spectroscopy. The possible structure of 2-naphthalenethiol orientations on gold

227

is modeled based on the features of the molecular arrangement shown in figure

13(c). The schematic structure with respect to underlying gold surface is shown

in figure 13(d) along with the dimensions. It can be seen that the surface

structure can be described as √3 x 3 R 300 overlayer structure of 2-

naphthalenethiol on gold.

Figure 13: (a) STM image of bare Au surface at 6nm x 6nm range. (b) Constant

height STM image of 2-naphthalenethiol adsorbed on Au surface at 6nm x 6nm

scan range showing individual molecules. (c) Zoomed image of 2.8nm x 2.8nm.

The slanted white line of 5Å corresponds to the distance between sulphur atoms

of adjacent columns of 2-naphthalenethiol molecules and vertical line of 8.4Å

corresponds to the distance between the two sulphur atoms of adjacent rows of

2-naphthalenethiol molecules. (d) Model of adsorption of 2-naphthalenethiol on

Au surface and its structural orientation of √ 3 X 3 R300 overlayer. The

arrangement of Au substrate atoms is shown in diamond shape on the right.

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II. SAMs OF THIOPHENOL AND AMINOTHIOPHENOLS ON Au

4.4. Introduction

We have carried out studies on the monolayer formation and its

characterization of some aromatic thiols namely thiophenol (TP), o-

aminothiophenol (o-ATP) and p-aminothiophenol (p-ATP) on polycrystalline

Au surface. We have evaluated the barrier property and blocking ability of these

aromatic monolayers on Au surface by studying the electron transfer reactions

of redox probe molecules on the SAM modified surfaces using electrochemical

techniques such as cyclic voltammetry and electrochemical impedance

spectroscopy. We have also studied the effect of pH on the blocking ability of

the monolayers of aminothiophenols on Au surface. It is found that the

monolayers of these aromatic thiols on Au surface exhibit a poor blocking

ability towards the redox reactions. We have tried to improve the barrier

property of these aromatic monolayers on Au surface by potential cycling and

mixed SAM formation using aliphatic thiols of comparable chain length.

We find from literature that there are a large number of reports on the

SAM of n-alkanethiols on gold surface [66-70], which is the most widely

studied system due to the flexibility offered by the long aliphatic chains, the

inertness of gold substrate and the ease of preparation method by a simple

immersion process [14]. Usually, the SAMs of aliphatic thiols are obtained by

immersing the metal into a dilute organic solution containing the thiol

molecules whose concentration varies from typically μM to mM in solvents

such as ethanol and acetonitrile. The SAMs of long-chain n-alkanethiols, which

exhibit a high degree of organization and long range ordering have been used as

model systems to study the electron transfer mechanism [71-75] and ω-

functionalized aliphatic thiols have been extensively used for modifying the

surface properties. The blocking ability of the monolayer is usually evaluated

229

by studying the electron transfer reactions of the redox probes like potassium

ferro/ferri cyanide and hexaammineruthenium(III) chloride on SAM modified

surfaces using electrochemical techniques namely cyclic voltammetry and

electrochemical impedance spectroscopy [61-63,76]. In contrast to the well-studied long chain aliphatic thiols used for SAM

formation, there is relatively a little attention that has been paid to the aromatic

thiols. In recent times, the research work on the SAM of aromatic thiols on Au

surface has attracted renewed interest due to their potential applications in

molecular electronics especially in the study of molecular wires and coulomb

blockade [77-81]. Aromatic thiol monolayers represent an interesting system

owing to the presence of delocalized π-electrons in the aromatic phenyl ring,

which results in increased electrical conductivity and the rigidity of aromatic

phenyl ring, which reduces the molecular flexibility. SAMs based on aromatic

thiols offer an opportunity to enhance the electrical coupling between the

electronic states of the electrode and the redox species in the solution. In

addition, it would be of great interest to study the electrode kinetics of SAM

modified electrodes using aromatic thiols. It is well known that the monolayers

of long chain aliphatic thiols offer a high resistance and slow down the

electrode kinetics significantly. On the other hand, the SAM of aromatic thiols

are expected to show a lower barrier to the electrode kinetics due to the

presence of high density of delocalized π-electrons in the phenyl ring. The

pioneering experiments on the adsorption of purely aromatic thiols were carried

out by Hubbard et al. [82], who studied the monolayer formation of thiophenol,

benzyl mercaptan and few other aromatic molecules on platinum [Pt (111)] and

silver [Ag (111)] using Auger electron spectroscopy (AES) and Electron energy

loss spectroscopy (EELS). These studies shown that the monolayers formed on

Pt (111) surface did not possess the long range ordering unlike that of Ag (111),

230

where they exhibit a long range order. Later, using vibrational spectroscopic

studies it has been shown that the aromatic molecules attach to the Au surface

through the thiolate bond formation and the results were helpful to determine

the orientation of molecules on the surface [83].

There are a few reports in literature on the SAM of thiophenol (TP)

and aminothiophenols (ATPs) on Au surface. Rubinstein et al. were the first to

investigate the formation of SAM based on aromatic thiols using

electrochemical techniques [84]. These authors studied the formation of SAMs

of thiophenol, biphenyl mercaptan and terphenyl mercaptan on gold surface and

found that the ordering, packing efficiency, stability and blocking ability of the

monolayers increased from thiophenol (TP) to terphenyl mercaptan [84]. There

are however a very few reports in literature on the electrochemistry of SAM of

p-aminothiophenol (p-ATP) on gold surface. Crooks et al. reported the

formation of SAM of p-aminothiophenol on Au surface and found that the

electron transfer reactions on the SAM modified surfaces were affected by the

pH of the medium [85]. The interesting aspect of this work is that the redox

reaction of hexaammineruthenium(III) chloride is facilitated even in acidic

medium and no mechanism for this unusual behaviour of SAM has been

proposed. Shannon et al. [86] reported the electrochemistry of mixed

monolayers of p-aminothiophenol (p-ATP) and thiophenol (TP) on Au surface

and proposed an ECE mechanism for the oxidation of SAM in acidic medium

resulting in the formation of dimer species, which has been proved by

spectroscopic techniques. Lukkari et al. reported the formation of SAM of p-

aminothiophenol on gold [87] and found that the potential for the surface

reaction of oxide stripping is shifted in the case of SAM modified electrodes in

acidic medium. These authors also proposed an ECE mechanism for the

formation of quinone derivative during dimerization reaction and the different

231

species formed during the oxidation reaction have been confirmed by XPS,

electro-reflectance spectroscopy and electrochemical techniques. A thorough

investigation on the electrochemistry and structure of the SAMs of isomers of

aminothiophenols on gold surface has been reported by Batz et al. [88]. These

authors found that the packing density of SAM varies in the order p-ATP>m-

ATP>o-ATP>TP and results in benzoquinone form during oxidation in acidic

medium, which has been confirmed by XPS. Mohri et al. [89,90] reported the

kinetic study on the monolayer formation of p-ATP on gold surface and also the

desorption of p-ATP bound to a gold surface using UV-absorption spectroscopy

and XPS.

While studies on the SAMs of thiophenol (TP) and aminothiophenols

(ATPs) on Au surface are available in literature [82-90], there is however, no

reported works on mixed SAMs using aliphatic thiols. The aliphatic thiols with

their small Van der Waals radii can easily pack the vacant regions occurring

due to pinholes and defects in the monolayer. This can provide effective barrier

to the access of redox species to the electrode surface. In this work we have also

tried to improve the blocking ability of TP and ATPs monolayers using two

different approaches namely, (i) the effect of potential cycling and (ii)

formation of mixed SAM using 1-Octanethiol and 1,6-Hexanedithiol. These

alkanethiols have comparable chain lengths with the thiophenol and

aminothiophenols. The effect of potential cycling on the structural aspects of

the monolayers on various surfaces had earlier been discussed and reported in

literature by other groups [31-33,91,92]. Anderson and Gatin studied the self-

assembled monolayer (SAM) of octanethiol on gold using polarization

modulation Fourier transform infrared reflection absorption spectroscopy (PM-

FTIRRAS) and found that the applied potential induces the structural changes

on the monolayer resulting in the formation of a compact and more organized

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monolayer [33]. Whitesides et al. studied the wetting of SAMs of

alkanethiolates on gold surface by applied electrical potentials using contact

angle measurements and found that it is possible to prepare patterned SAMs in

which certain regions can be transformed from hydrophobic to hydrophilic

nature by changing the electrode potential [91]. Boubour and Lennox reported

the stability of ω-functionalized SAMs as a function of applied potential [32]

and the potential induced defects in n-alkanethiol monolayers [31] by studying

the ionic permeability using electrochemical impedance spectroscopy. There are

several reports in literature on the study of electron transfer reactions through

mixed monolayers [93-96]. The replacement of one thiol by another thiol

moiety was also reported. Even though there are quite a few reports on the

electrochemical aspects of SAM of TP and ATPs, we find from literature that

the electron transfer reactions of redox couples, the effect of pH and a clear

understanding of pinholes and defects, determination of surface coverage on the

SAM modified surfaces have not been clearly resolved and reported. We have

also used a mixed aromatic-aliphatic thiol monolayer in order to improve the

blocking ability of the SAM, essentially by filling up the pinholes and defects

present within the monolayer.

In this section, we discuss the results of our study on the

electrochemistry of self-assembled monolayers of thiophenol (TP), o-

aminothiophenol (o-ATP) and p-aminothiophenol (p-ATP) on gold surface. We

have also described the effect of potential cycling and mixed SAM formation

on the blocking property of these monolayers on Au surface. Electrochemical

techniques such as cyclic voltammetry (CV), electrochemical impedance

spectroscopy (EIS) and gold oxide stripping analysis were employed to evaluate

the barrier properties of the monolayer. We have used two important redox

systems namely [Fe(CN)6]3-|4- and [Ru(NH3)6]2+|3+ to study the redox reactions

233

on the SAM modified electrodes. The effect of pH on the blocking ability of

these monolayers has also been investigated using CV and EIS by studying the

electron transfer reactions of the redox probes on the SAM modified surfaces

under the acidic and basic conditions. Impedance spectroscopy data were used

to determine the charge transfer resistance (Rct) value, which is the measure of

blocking ability of the monolayer towards the diffusion of redox species. In

addition, EIS data are used for the determination of surface coverage and

pinholes and defects analysis.

4.5. Experimental section

4.5.1. Chemicals

All the chemical reagents used in this study were analytical grade (AR)

reagents. Thiophenol (Aldrich), o-Aminothiophenol (Aldrich), p-

Aminothiophenol (Aldrich), 1-Octanethiol (Aldrich), 1,6-Hexanedithiol

(Aldrich), ethanol (99.95%; Merck), potassium ferrocyanide (Loba), potassium

ferricyanide (Qualigens), hexaammineruthenium(III) chloride (Alfa Aesar),

sodium fluoride (Qualigens), lithium perchlorate (Acros Organics) and

perchloric acid (Qualigens) were used in this study as received. Millipore water

having a resistivity of 18 MΩ cm was used to prepare the aqueous solutions.

4.5.2. Fabrication of electrodes and electrochemical cell

We have carried out the monolayer adsorption, preparation of mixed

SAM and its electrochemical characterization using a gold wire electrode. Gold

sample of purity 99.99% was obtained from Arora Mathey, India. The gold wire

electrode was fabricated by proper sealing of a 99.99% pure gold wire of

0.5mm diameter with soda lime glass having a thermal expansion coefficient

close to that of gold. The electrical contact has been provided through a copper

234

wire. This planar disc electrode has a geometric area of 0.002 cm2. This small

area-working electrode has a highly reproducible true surface area even after

repeated usage, which has been confirmed by the measurement of roughness

factor of the gold wire electrode by potential cycling in 0.1M perchloric acid. A

conventional three-electrode electrochemical cell was used for the experimental

studies involving the characterization of SAM modified electrodes using

electrochemical techniques. A platinum foil of large surface area was used as a

counter electrode and a saturated calomel electrode (SCE) was used as a

reference electrode while the SAM modified electrodes were used as the

working electrode. The cell was cleaned thoroughly before each experiment and

kept in a hot-air oven at 1000C for at least 1 hour before the start of the

experiment.

4.5.3. Electrode pre-treatment and preparation of aromatic SAMs and

mixed SAMs on gold surface

Immediately before use, the gold wire electrode was polished with

emery papers of grade 800 and 1500 respectively, followed by polishing in

aqueous slurries of progressively finer alumina of sizes ranging from 1.0μm,

0.3μm to 0.05 μm on a microcloth. After this, the electrode was ultrasonicated

in millipore water to remove alumina particles. Then it was cleaned in a

“piranha” solution, which is a mixture of 30% H2O2 and concentrated H2SO4 in

1:3 ratio. Finally, the polished gold wire electrode was thoroughly cleaned and

rinsed in distilled water, followed by rinsing in millipore water before SAM

formation. The monolayers were prepared by keeping the Au wire electrode in

neat thiol (in the case of TP and o-ATP) and 20mM thiol solution in ethanol (in

the case of p-ATP) for about 1 hour. We have found from our earlier work that

good surface coverage is obtained in view of the high concentration of thiols

235

used for the monolayer preparation [97]. After the adsorption of thiol, the

electrode was rinsed with ethanol, distilled water and finally with millipore

water and immediately used for the analysis. We have used two different thiols

for mixed SAM formation with aromatic thiols namely, 1-Octanethiol (neat

thiol) and 1,6-Hexanedithiol (neat thiol), which have comparable chain lengths

to that of aromatic thiols used for the monolayer formation on Au surface. In

the case of mixed SAM formation, initially the gold wire electrode was dipped

in aromatic thiols for about 1 hour. After the adsorption of thiol, the electrode

was rinsed with ethanol followed by cleaning in distilled water and immediately

kept in neat thiol of either 1-Octanethiol or 1,6-Hexanedithiol for about 1 hour.

On completion of mixed SAM formation, the monolayer-coated electrode was

rinsed with ethanol; cleaned in distilled water and finally rinsed with millipore

water and used for the analysis immediately. The mixed SAM formation was

carried out in the following way viz., (a) 1-Octanethiol with TP or o-ATP or p-

ATP on Au surface and (b) 1,6-Hexanedithiol with TP or o-ATP or p-ATP on

Au surface.

4.5.4. Electrochemical characterization

Cyclic voltammetry and electrochemical impedance spectroscopy were

used for the electrochemical characterization of monolayer coated and mixed

SAM modified electrodes. The barrier property of the monolayer has been

evaluated by studying the electron transfer reaction using two different redox

probes namely, potassium ferrocyanide (negative redox probe) and

hexaammineruthenium(III) chloride (positive redox probe). Cyclic voltammetry

was performed in aqueous solutions of 10mM potassium ferrocyanide in 1M

NaF at a potential range of –0.1V to +0.5V vs. SCE and 1mM

hexaammineruthenium(III) chloride in 0.1M lithium perchlorate at a potential

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range of –0.45V to +0.1V vs. SCE. The impedance measurements were carried

out using an alternating current (ac) of a 10mV amplitude signal at a formal

potential of the redox couple using a wide frequency range of 100kHz to 0.1Hz,

in solution containing always equal concentrations of both the oxidized and

reduced forms of the redox couple namely, 10mM potassium ferrocyanide and

10mM potassium ferricyanide in 1M NaF as the supporting electrolyte. The

gold oxide stripping analysis was conducted in 0.1M perchloric acid in the

potential range from 0.2V to 1.4V vs. SCE. The packing density and ordering of

the monolayer can be understood by measuring the charge under the cathodic

Au oxide stripping peak and comparing with that of the bare Au electrode. The

pH of the medium was adjusted using 0.1M H2SO4 and 0.5M NaOH in order to

maintain the respective acidic and basic conditions to study the redox reactions

on the SAM modified electrodes.

In order to analyze the effect of potential cycling on the barrier

property and structural organization of the monolayer on Au surface, the

following experiments were carried out. Immediately after the monolayer

formation, the electrode is subjected to potential cycling between 0.0V and

0.6V vs. SCE in a solution containing purely supporting electrolyte without any

redox species and the same electrode is subsequently used for the cyclic

voltammetry and impedance spectroscopic studies to determine the barrier

property of the monolayer. From the impedance data, the charge transfer

resistance (Rct) was determined using the equivalent circuit fitting analysis. In

addition, EIS data were used for the analysis of pinholes and defects in the

cases of monolayer and mixed monolayer modified electrodes. From the Rct

values and pinholes and defects analysis, the surface coverage (θ) values of the

monolayer and mixed monolayer on Au surface were also determined. All the

experiments were performed at room temperature of 250C.

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4.5.5. Instrumentation

Cyclic voltammetry was carried out using an EG&G potentiostat

(model 263A) interfaced to a computer through a GPIB card (National

Instruments). The potential ranges and scan rates used are shown in the

respective diagrams. For electrochemical impedance spectroscopy

measurements the potentiostat was used along with an EG&G 5210 lock-in-

amplifier controlled by Power Sine software. The impedance spectroscopy data

were used for the equivalent circuit fitting analysis using Zsimpwin software

(EG&G) developed on the basis of Boukamp’s model. Based on this procedure,

the charge transfer resistance values (Rct) of SAM modified electrodes were

calculated, from which the surface coverage values of the monolayers on Au

surface were also determined.

4.6. Results and discussion

4.6.1. Using aromatic SAM modified electrodes

4.6.1.1. Cyclic voltammetry

Cyclic voltammetry is an important tool to assess the quality of the

monolayer and its blocking ability by studying the electron transfer reaction on

the SAM modified surfaces using diffusion controlled redox couple as probe

molecule. The SAMs of TP, o-ATP and p-ATP were formed on the

polycrystalline Au surface by dipping the Au wire electrode in the respective

thiol solutions. Figure 14 shows the cyclic voltammograms of bare Au electrode

and SAM modified electrodes in 10mM potassium ferrocyanide with 1M NaF

as the supporting electrolyte at a potential scan rate of 50mV/s. It can be seen

from inset of the figure that the bare Au electrode shows a perfect reversible

voltammogram for the redox couple indicating that the electron transfer

reaction is completely under diffusion control process. Figures 14(a), (b) and (c)

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show the respective cyclic voltammograms of SAMs of TP, o-ATP and p-ATP

in the same solution. In contrast to the bare Au electrode, the SAM modified

electrodes in the cases of TP and o-ATP do not show any peak formation

implying that the redox reaction is inhibited. It can be seen from figure 14(a)

that the SAM of TP on Au electrode shows ‘s’ shaped curve with a large current

separation between the forward and reverse direction indicating the moderate

blocking behaviour of the monolayer with a large number of pinholes and

defects. This observation is at variance with the results reported by Rubinstein

et al. [84]. These authors reported that the monolayer of TP on Au is hardly

blocking the electron transfer reaction. The difference in the blocking ability of

the SAM of TP in the present case is attributed to the longer time of adsorption

(1 hour) and the use of neat thiol for monolayer formation, in contrast to 15

minutes adsorption in 1mM aqueous solution of TP in the other case [84]. In the

case of o-ATP (Fig. 14(b)) the CV shows a complete blocking to the electron

transfer reaction implying the formation of highly compact and ordered

monolayer on Au surface. On the other hand, SAM of p-ATP shows a quasi-

reversible behaviour for the redox reaction exhibiting a rather imperfect

blocking ability. This implies the formation of a highly disorganized and

disordered poor monolayer film in the case of p-ATP on Au surface.

Figure 15 shows the comparison of cyclic voltammograms of bare Au

electrode and SAM modified electrodes in 1mM hexaammineruthenium(III)

chloride with 0.1M LiClO4 as the supporting electrolyte at a potential scan rate

of 50mV/s. It can be seen from inset of the figure that the bare Au electrode

shows a reversible voltammogram implying the diffusion-controlled process for

the redox reaction. Figures 15(a), (b) and (c) show the cyclic voltammograms of

SAMs of TP, o-ATP and p-ATP in the same solution respectively. It can be

noted from the figures that the redox reaction of ruthenium complex is not

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inhibited significantly in all the cases although the SAM of TP on Au exhibits a

very poor blocking behaviour. The results obtained in the case of SAM of p-

ATP on Au surface is similar to the report of Crooks et al. [85], where they

found that the redox reaction of ruthenium complex is quite facile for p-ATP

modified Au electrode. These experiments lead to an interesting observation

that the aromatic SAM is selective to ruthenium redox reaction and not to

ferrocyanide redox reaction.

Figure 14: Cyclic voltammograms of SAMs of (a) Thiophenol (TP), (b) o-

Aminothiophenol (o-ATP) and (c) p-Aminothiophenol (p-ATP) on Au surface in

10mM potassium ferrocyanide with 1M NaF as a supporting electrolyte at a

potential scan rate of 50mV/s. Inset shows the cyclic voltammogram of a bare

Au electrode in the same solution for comparison.

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Figure 15: Cyclic voltammograms in 1mM hexaammineruthenium(III) chloride

with 0.1M LiClO4 as a supporting electrolyte at a potential scan rate of 50mV/s

for the SAMs of (a) Thiophenol (TP), (b) o-Aminothiophenol (o-ATP) and (c) p-

Aminothiophenol (p-ATP) on Au electrode. For comparison the cyclic

voltammogram of bare Au electrode in the same solution is shown in the inset

of the figure.

4.6.1.2. Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy is a powerful technique to

evaluate the structural integrity of the monolayer and to determine the charge

transfer resistance of the SAM modified electrodes against the diffusion of the

redox probe in a quantitative manner. In addition, the impedance data are useful

in the determination of the surface coverage and other kinetic parameters of the

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monolayer coated electrodes. Figure 16A shows the impedance plot (Nyquist

plot) of the bare Au electrode in equal concentrations of potassium ferro/ferri

cyanide solution with NaF as the supporting electrolyte. Figure 16B shows the

same plot for the SAM modified electrodes. It can be seen from the figure 16A

that the bare Au electrode shows a low frequency straight line with a very small

semicircle at high frequency region indicating that the electron transfer process

is essentially diffusion controlled. On the other hand, SAM modified electrodes

in the cases of TP (Fig. 16B(a)) and o-ATP (Fig. 16B(b)) show the formation of

semicircle implying a good blocking behaviour and charge transfer control for

the electron transfer reaction. It can be noted from figure 16B(a) that the

impedance values are almost constant at lower frequencies in the case of SAM

of TP on Au indicating the diffusion of probe molecules through the pores and

pinholes. In contrast, SAM of p-ATP on Au (Fig. 16B (c)) shows a straight line

at low frequency and a very small semicircle at high frequency region similar to

that of bare Au implying the poor blocking ability of the SAM towards the

redox reaction.

Figures 17A and B show the respective Nyquist plots of bare Au

electrode and SAM modified electrodes in 1mM hexaammineruthenium(III)

chloride solution with 0.1M LiClO4 as the supporting electrolyte. The

impedance plot of bare Au electrode (Fig. 17A) exhibits a low frequency

straight line with a very small semicircle formation at high frequency indicating

the diffusion-controlled process for the electron transfer reaction of ruthenium

complex.

242

Figure 16: Impedance plots in 10mM potassium ferrocyanide and 10mM

potassium ferricyanide with 1M NaF as supporting electrolyte for, (A) Bare Au

electrode and (B) SAMs of TP (a), o-ATP (b) and p-ATP (c) on Au surface.

243

Figure 17: Impedance plots in 1mM hexaammineruthenium(III) chloride with

0.1M LiClO4 as supporting electrolyte for, (A) Bare Au electrode and (B) SAMs

of TP (a), o-ATP (b) and p-ATP (c) on Au electrode.

244

Similarly, the SAM modified electrodes in the cases of o-ATP (Fig.

17B(b)) and p-ATP (Fig. 17B(c)) show the facile nature of the Ru(III) electron

transfer reaction indicating a very poor blocking ability of the monolayer. It can

be seen from figure 17B(a) that the SAM of TP on Au shows a formation of

semicircle at high frequency region and a straight line at low frequency with

small increase in the charge transfer resistance implying the quasi-reversible

behaviour for the redox reaction showing the poor blocking ability of the

monolayer. These results are in conformity with our observations using cyclic

voltammetric studies discussed earlier.

4.6.1.3. Gold oxide stripping analysis

The qualitative analysis of surface coverage of the monolayer can be

carried out using oxide-stripping method. The cyclic voltammogram of bare Au

electrode in 0.1M HClO4 shows the usual features of gold oxide formation

during the positive potential scanning and oxide stripping upon reversing the

scan direction. The quantity of electric charges passed during its removal of

oxide is proportional to the effective electrode area. By comparing the charge of

a SAM modified electrode with the respective charge of the bare gold electrode,

the surface coverage of the monolayer can be determined qualitatively.

Figure 18 shows the cyclic voltammograms of different electrodes in

0.1M HClO4 for the gold oxide stripping analysis. For comparison the cyclic

voltammogram of bare Au electrode was also given in figure 18(a) and it shows

the gold oxide formation and stripping peaks. Figures 18(b), (c) and (d) show

the respective cyclic voltammograms of SAMs of TP, o-ATP and p-ATP on Au

surface in 0.1M HClO4 solution. Compared with the bare Au electrode, the

measured charges of the SAM modified electrodes are significantly less, as

expected. The area fraction of the coverage is calculated using the formula (1–

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QSAM/QBare Au), where QSAM and QBare Au are the charges measured for the SAM

modified electrodes and the bare Au electrode respectively.

Figure 18: Cyclic voltammograms of gold oxide stripping analysis in 0.1M

HClO4 for, (a) bare Au electrode, (b) SAM of TP on Au electrode, (c) SAM of o-

ATP on Au surface and (d) SAM of p-ATP on Au electrode.

It can be seen from the figures 18(b) and (c) that the charge values

measured from the cathodic stripping peaks for the SAMs of TP and o-ATP are

almost the same (~350 nC/cm2) and a surface coverage value of 80% has been

obtained, which is higher than the surface coverage value of 31% reported for

TP on gold surface [84]. In the case of SAM of p-ATP (Fig. 18(d)), the

calculated charge value is quite high, which results in a very low surface

coverage value of 55%. The fact that the surface coverage values determined

246

using gold oxide stripping analysis are lower can be attributed to the smaller

size of OH− ion which acts as a probe in the present study that can have an easy

access to the electrode surface through the pinholes and defects when compared

to that of bulkier ions. However, after repeated cycles in perchloric acid, the

SAM modified electrodes show the cyclic voltammogram similar to that of bare

Au electrode indicating the complete removal of the monolayer. This confirms

the fact that the monolayer is not electrochemically stable when cycled to high

positive potentials.

4.6.1.4. Effect of pH on the blocking property of aromatic SAMs on Au

We have investigated the effect of pH on the blocking ability of SAMs

of o-ATP and p-ATP on Au surface. Due to the presence of amino group in

these molecules, depending upon the pH of the medium (acidic or basic), the

surface modification of SAMs to hydrophilic or hydrophobic nature can be

obtained. We have performed the study of electron transfer reaction of redox

probe molecules on the SAM modified surfaces in acidic (pH = 3.2 to 3.5) and

basic (pH = 11.0 to 11.2) conditions. The pH of the medium was adjusted using

0.1M H2SO4 and 0.5M NaOH aqueous solutions.

Cyclic voltammetry

(a). Acidic condition

Figure 19A shows the cyclic voltammograms of SAM modified

surfaces in 10mM potassium ferrocyanide with 1M NaF as a supporting

electrolyte at a potential scan rate of 50mV/s under the acidic condition. It can

be seen from the figure that both the SAMs of o-ATP (Fig. 19A (a)) and p-ATP

(Fig. 19A (b)) on Au surface show the facile kinetics of the electron transfer

reaction, implying a very poor blocking ability of the monolayer. Figure 19B

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shows the cyclic voltammograms of the monolayer-coated electrodes in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as a supporting

electrolyte at a potential scan rate of 50mV/s under the acidic condition. Figures

19B (a) and (b) show the respective cyclic voltammograms of SAMs of o-ATP

and p-ATP on Au surface for the redox reaction of ruthenium complex.

Surprisingly, the electron transfer reaction is allowed in both the cases showing

the facile nature of Ru(III) reaction on the SAM modified surfaces indicating

the poor blocking behaviour of the monolayer. The results obtained are similar

to the report of Crooks et al. [85], where they found that the redox reaction of

ruthenium complex is completely allowed in the case of SAM of p-ATP on Au

surface under the acidic condition and no mechanism for this unusual behaviour

of the monolayer is proposed.

(b). Basic condition

Figure 20A shows the cyclic voltammograms of monolayer-coated

surfaces in 10mM potassium ferrocyanide with 1M NaF as a supporting

electrolyte at a potential scan rate of 50mV/s under the basic condition. It can

be seen from the figure 20A (a) that the SAM of o-ATP on Au shows the good

blocking ability to the redox reaction indicating the formation of a highly dense

and ordered hydrophobic SAM of o-ATP. In contrast, the SAM of p-ATP on

Au (Fig. 20A (b)) does not inhibit the electron transfer reaction implying the

formation of a poorly ordered SAM under the basic condition. Figure 20B

shows the cyclic voltammograms of SAM modified surfaces in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as a supporting

electrolyte at a potential scan rate of 50mV/s under the basic condition. It can

be seen from the figure that both the SAMs of o-ATP (Fig. 20B (a)) and p-ATP

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(Fig. 20B (b)) on Au surface show a facile kinetics for the ruthenium redox

reaction, indicating the poor blocking ability of SAMs.

Figure 19: (A) Cyclic voltammograms in 10mM potassium ferrocyanide with

1M NaF as a supporting electrolyte at a potential scan rate of 50mV/s under

the acidic condition (pH = 3.2 – 3.5) for, (a) SAM of o-ATP on Au surface and

(b) SAM of p-ATP on Au surface. (B) Cyclic voltammograms in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as a supporting

electrolyte at a potential scan rate of 50mV/s under the acidic condition (pH =

3.2 – 3.5) for, (a) SAM of o-ATP on Au electrode and (b) SAM of p-ATP on Au

electrode.

249

Figure 20: Cyclic voltammograms in 10mM potassium ferrocyanide with 1M

NaF as a supporting electrolyte at a potential scan rate of 50mV/s under the

basic condition (pH = 11.0 – 11.2) for, (a) SAM of o-ATP on Au surface and (b)

SAM of p-ATP on Au surface. (B) Cyclic voltammograms in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as a supporting

electrolyte at a potential scan rate of 50mV/s under the basic condition (pH =

11.0 – 11.2) for, (a) SAM of o-ATP on Au electrode and (b) SAM of p-ATP on

Au electrode.

250

Electrochemical impedance spectroscopy

(a). Acidic condition

Figure 21A shows the impedance (Nyquist) plots of SAM modified

surfaces in 10mM potassium ferrocyanide and 10mM potassium ferricyanide

with 1M NaF as a supporting electrolyte under the acidic condition. It can be

seen from the figure that the SAM of o-ATP on Au (Fig. 21A (a)) shows the

formation of a small semicircle at high frequency region and a straight line at

low frequency indicating the poor blocking ability of the monolayer. Similarly,

the SAM p-ATP on Au (Fig. 21A (b)) shows the formation of straight line in

the entire range of frequency used for the study implying the diffusion-

controlled process for the electron transfer reaction. Figure 21B shows the

Nyquist plots of the monolayer coated electrodes in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as a supporting

electrolyte under the acidic condition. It can be noted from the figure that both

the SAMs of o-ATP (Fig. 21B (a)) and p-ATP (Fig. 21B (b)) on Au surface

show the facile kinetics for the ruthenium electron transfer reaction implying

the poor blocking ability of the monolayer under the acidic condition.

(b). Basic condition

Figure 22A shows the typical impedance plots of SAM modified

electrodes in 10mM potassium ferrocyanide and 10mM potassium ferricyanide

with 1M NaF as a supporting electrolyte under the basic condition. It can be

seen from the figure 22A (a) that the SAM of o-ATP on Au exhibits a

semicircle indicating the good blocking behaviour of SAM to the redox

reaction, while the SAM of p-ATP on Au (Fig. 22A (b)) shows a smaller

semicircle and a diffusion controlled straight line at low frequency implying a

less blocking behaviour of the SAM under the basic condition.

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Figure 21: Impedance plots in equal concentrations of both potassium

ferrocyanide and potassium ferricyanide with NaF as a supporting electrolyte

under the acidic condition (pH = 3.2 – 3.5) for, (a) SAM of o-ATP on Au

electrode and (b) SAM of p-ATP on Au electrode. (B) Impedance plots in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as a supporting

electrolyte under the acidic condition (pH = 3.2 – 3.5) for, (a) SAM of o-ATP

on Au surface and (b) SAM of p-ATP on Au surface.

252

Figure 22: Impedance plots in equal concentrations of both potassium

ferrocyanide and potassium ferricyanide with NaF as a supporting electrolyte

under the basic condition (pH = 11.0 – 11.2) for, (a) SAM of o-ATP on Au

electrode and (b) SAM of p-ATP on Au electrode. (B) Impedance plots in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as a supporting

electrolyte under the basic condition (pH = 11.0 – 11.2) for, (a) SAM of o-ATP

on Au surface and (b) SAM of p-ATP on Au surface.

253

Figure 22B shows the Nyquist plots of the monolayer coated electrodes

in 1mM hexaammineruthenium(III) chloride with 0.1M LiClO4 as a supporting

electrolyte under the basic condition. It can be seen from the figure 22B (a) that

the SAM o-ATP on Au shows a very small semicircle at high frequency region

and a straight line at low frequency region indicating a very poor blocking

ability of SAM towards the redox reaction of ruthenium complex. Similarly,

SAM of p-ATP on Au (Fig. 22B (b)) shows a straight-line formation in the

entire range of frequency implying the facile kinetics of ruthenium redox

reaction on the SAM modified surface under the basic condition. These results

are in conformity with our CV results under the similar acidic and basic

conditions. From the experiments we have observed that the SAMs are selective

to ruthenium redox reaction even under the acidic condition, an aspect that will

be discussed later.

4.6.2. Effect of potential cycling

We find from the cyclic voltammetry and electrochemical impedance

spectroscopic studies discussed earlier that the self-assembled monolayers of

thiophenol (TP), o-aminothiophenol (o-ATP) and p-aminothiophenol (p-ATP)

on gold surface form stable and reproducible but moderately blocking

monolayers. From the experimental results, we concluded that these monolayers

have a large number of pinholes and defects, which accounts for the poor

blocking ability of these monolayers. We have tried to improve the blocking

behaviour of these monolayer-modified electrodes by two different processes,

namely by subjecting the freshly formed SAM electrodes to (i) potential cycling

in a pure supporting electrolyte without any redox species and (ii) mixed SAM

formation with either 1-Octanethiol or 1,6-Hexanedithiol. Immediately after the

SAM formation of respective aromatic thiols on Au surface, the monolayer-

254

coated electrodes are potential cycled in the positive potential range from 0.0V

to 0.6V vs. SCE in 1M NaF aqueous solution without any redox species. After

the potential cycling, the same electrode is used for the cyclic voltammetric and

electrochemical impedance spectroscopic studies to evaluate the barrier

property of the monolayers.

4.6.2.1. Cyclic voltammetry

Figure 23 shows the cyclic voltammograms of bare Au electrode, SAM

modified electrodes and potential cycled electrodes (after the SAM formation)

in 10mM potassium ferrocyanide with 1M NaF as the supporting electrolyte at

a potential scan rate of 50mV/s. It can be seen from figure 23A that the bare Au

electrode shows a perfect reversible voltammogram for the redox couple

indicating that the electron transfer reaction is completely under diffusion

control process. Figure 23B shows the cyclic voltammograms of (a) SAM of TP

on Au surface and (b) potential cycled SAM modified electrode in the same

solution. In contrast to bare Au electrode, the SAM modified and potential

cycled electrodes do not show any peak formation implying that the redox

reaction is inhibited. It can be noted from figure 23B (b) that the potential

cycled electrode shows a significantly less current when compared to SAM

modified electrode indicating a possible structural reorganization of monolayer,

which causes a reduction in the number of pinholes and defects. The CVs show

‘s’ shaped curve with a large current separation between the forward and

reverse scans implying a microelectrode array type behaviour [98-100] in which

the pinholes are distributed randomly over the surface. Figure 23C shows the

comparison of cyclic voltammograms of SAM of o-ATP on Au surface (a) and

potential cycled monolayer-coated electrode (b). It can be seen from figure 23C

that the SAM of o-ATP on Au surface shows a good blocking behaviour to the

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electron transfer reaction. The potential cycled electrode (after SAM formation)

(Fig. 23C (b)) shows a significant decrease in the current value when compared

to the SAM modified electrode (Fig. 23C (a)) indicating that the blocking

ability of the monolayer is improved to a large extent after the potential cycling.

Figure 23D shows the CVs of SAM of p-ATP on Au electrode (a) and the

potential cycled electrode (b). It can be seen from Fig. 23D (a) that the SAM of

p-ATP on Au surface shows a quasi-reversible behaviour for the redox reaction

implying a poor blocking ability of the monolayer. Although there is a small

decrease in the current after the potential cycling, the monolayer exhibits a

rather imperfect blocking behaviour. It can be observed that in all these cases of

SAMs of aromatic thiols on Au surface, the blocking ability of the monolayer

has improved significantly after the potential cycling. This can only be

attributed to structural modification and more compact organization of the

monolayer leading to a large reduction in the number of pinholes and defects.

Figure 24 shows the cyclic voltammograms of bare Au electrode,

monolayer-coated electrodes and potential cycled electrodes in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as the supporting

electrolyte at a potential scan rate of 50mV/s. It can be seen from figure 24A

that the bare Au electrode shows a reversible behaviour for the redox reaction

indicating that the electron transfer process is under diffusion control. Figures

24B (a) and (b) show the CVs of the SAM of TP on Au surface and the

potential cycled electrode after the SAM formation respectively. It can be seen

from the figure that the redox reaction of ruthenium complex is not inhibited

significantly in the case of SAM of TP even after potential cycling, implying a

rather poor blocking property of the monolayer. Figures 24C (a) and (b) show

the comparison of cyclic voltammograms of the SAM of o-ATP on Au surface

and the corresponding potential cycled electrode. It can be noted from the figure

256

that the SAM of o-ATP on Au shows a considerable blocking to the ruthenium

redox reaction after the potential cycling, though it is not totally inhibited.

Figure 23: Cyclic voltammograms in 10mM potassium ferrocyanide with 1M

NaF as a supporting electrolyte at a potential scan rate of 50mV/s for, (A) Bare

Au electrode. (B) SAM of thiophenol (TP) on Au electrode (a), a potential

cycled electrode after the SAM formation of TP on Au (b). (C) SAM of o-

aminothiophenol (o-ATP) on Au electrode (a), a potential cycled electrode after

the SAM formation of o-ATP on Au (b). (D) SAM of p-aminothiophenol (p-ATP)

on Au electrode (a), a potential cycled electrode after the SAM formation of p-

ATP on Au (b).

257

Figure 24: Cyclic voltammograms in 1mM hexaammineruthenium(III) chloride

with 0.1M LiClO4 as a supporting electrolyte at a potential scan rate of 50mV/s

for, (A) Bare gold electrode. (B) SAM of thiophenol (TP) on Au electrode (a), a

potential cycled electrode after the SAM formation of TP on Au (b). (C) SAM of

o-aminothiophenol (o-ATP) on Au electrode (a), a potential cycled electrode

after the SAM formation of o-ATP on Au (b). (D) SAM of p-aminothiophenol (p-

ATP) on Au electrode (a), a potential cycled electrode after the SAM formation

of p-ATP on Au (b).

258

Figures 24D (a) and (b) show the CVs of the SAM of p-ATP on Au

surface and the corresponding potential cycled electrode (after the SAM

formation). It can be observed that the redox reaction of ruthenium complex is

not at all inhibited as indicated by the reversible nature of the cyclic

voltammograms. Thus shows the facile nature of the electron transfer process

with the redox reaction under diffusion control. Even after the potential cycling,

the SAM modified electrode does not show any improvement in the blocking

behaviour. It can be noticed from the figures that the SAM modified electrodes

and the corresponding potential cycled electrodes hardly block the redox

reaction of ruthenium complex, which is very much in contrast to the behaviour

of ferrocyanide | ferricyanide redox couple.

4.6.2.2. Electrochemical impedance spectroscopy

Figure 25 shows the impedance plots (Nyquist plots) of the bare gold

electrode, SAM modified electrodes and the potential cycled electrodes (after

the SAM formation) in equal concentrations of potassium ferrocyanide and

potassium ferricyanide aqueous solution with 1M NaF as the supporting

electrolyte. Figure 25A shows a straight line at low frequency and a very small

semicircle at high frequency region indicating that the electron transfer process

is essentially under diffusion control for the redox couple on the bare Au

electrode. Figures 25B (a) and (b) show the respective impedance plots of SAM

of TP on Au surface and the corresponding potential cycled electrode. In

contrast to bare Au electrode, SAM of TP on Au and the corresponding

potential cycled electrode show a good blocking behaviour of the monolayer

with a well-defined semicircle at higher frequency region. It is well known that

the charge transfer resistance (Rct) can be determined from the intercept of

semicircle on the Z′ axis. It can be seen from the figure that the Rct value of

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potential cycled electrode (Fig. 25B (b)) is significantly increased when

compared to the monolayer-coated electrode (Fig. 25B (a)). This confirms an

improvement in blocking ability of the monolayer after the potential cycling.

Similar behaviour was observed for the SAM of o-ATP on Au surface (Fig.

25C (a)) and the corresponding potential cycled electrode (Fig. 25C (b)). The

better blocking property of the monolayer of o-ATP on Au after the potential

cycling is manifested by the increase in the charge transfer resistance, Rct value.

On the other hand, it can be seen from the Nyquist plots of figure 25D that the

monolayer of p-ATP on Au surface and the corresponding potential cycled

electrodes do not significantly block the redox reaction. In this case the Nyquist

plot shows a straight line at low frequency region implying that the electron

transfer process is under diffusion control. In other words the SAM of p-ATP

on Au surface shows a poor blocking behaviour although there is a marginal

increase in the Rct value after the potential cycling.

Figure 26 shows the impedance plots (Nyquist plots) of the bare Au

electrode, SAM modified electrodes and the potential cycled electrodes in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as the supporting

electrolyte. It can be seen from the figure 26A that the bare Au electrode

exhibits a low frequency straight line with a very small semicircle at high

frequency region indicating the diffusion-controlled process for the redox

reaction of ruthenium complex. Figures 26B,C and D show the respective

impedance plots of SAM of TP, o-ATP and p-ATP on Au surface and their

corresponding potential cycled electrodes in the same solution. It can be noticed

from the figures of the SAM modified electrodes and their corresponding

potential cycled electrodes that the Ru(III) electron transfer reaction is facile

though there is a marginal increase in Rct value in the latter case. It is worth

recalling that these SAM modified electrodes and their corresponding potential

260

cycled electrodes show a very good blocking ability to ferrocyanide |

ferricyanide redox reaction, an aspect which will be discussed later. It is worth

pointing out that these results are in conformity with our observations using

cyclic voltammetric studies described earlier.

Figure 25: Impedance plots in an aqueous solution of 10mM potassium

ferrocyanide and 10mM potassium ferricyanide with 1M NaF as a supporting

electrolyte for, (A) Bare Au electrode. (B) SAM of thiophenol (TP) on Au

electrode (a), a potential cycled electrode after the SAM formation of TP on Au

(b). (C) SAM of o-aminothiophenol (o-ATP) on Au electrode (a), a potential

cycled electrode after the SAM formation of o-ATP on Au (b). (D) SAM of p-

aminothiophenol (p-ATP) on Au electrode (a), a potential cycled electrode after

the SAM formation of p-ATP on Au (b).

261

Figure 26: Impedance plots in 1mM hexaammineruthenium(III) chloride with

0.1M LiClO4 as a supporting electrolyte for, (A) Bare Au electrode. (B) SAM of

thiophenol (TP) on Au electrode (a), a potential cycled electrode after the SAM

formation of TP on Au (b). (C) SAM of o-aminothiophenol (o-ATP) on Au

electrode (a), a potential cycled electrode after the SAM formation of o-ATP on

Au (b). (D) SAM of p-aminothiophenol (p-ATP) on Au electrode (a), a potential

cycled electrode after the SAM formation of p-ATP on Au (b).

262

4.6.2.3. Gold oxide stripping analysis

The surface coverage of these monolayers on Au surface can be

determined qualitatively using gold oxide stripping method. The cyclic

voltammogram of bare gold electrode in 0.1M HClO4 shows the usual features

of gold oxide formation during the positive potential scanning and oxide

stripping upon reversal of the scan direction. The quantity of electric charges

passed during the removal of oxide is proportional to the effective electrode

area. By comparing the charge of a SAM modified electrode and its

corresponding potential cycled electrode with the respective charge of the bare

gold electrode, the surface coverage of the monolayer can be determined in a

qualitative manner.

Figure 27 shows the cyclic voltammograms of gold oxide formation

and stripping for the SAM modified electrodes and the potential cycled

electrodes in 0.1M HClO4 aqueous solution. For comparison the cyclic

voltammogram of bare Au electrode was also shown in Fig. 27A and it shows

the respective gold oxide formation and stripping peaks. Figures 27B, C and D

show the cyclic voltammograms of SAMs of TP, o-ATP and p-ATP on Au

surface and their corresponding potential cycled electrodes in 0.1M HClO4

aqueous solution respectively. It can be seen from the figures that the measured

charges of the monolayer-coated electrodes are significantly less when

compared to the charge of bare Au electrode, which is as expected. In the cases

of SAMs of TP (Fig. 27B) and o-ATP (Fig. 27C) on Au surface, the charge

values are not significantly affected even after the potential cycling. However,

the more horizontal base line in the cases of potential cycled electrodes

indicates that the ionic permeability of the monolayer is uniform throughout the

scanned potential range. This implies that potential cycling has incorporated the

structural changes of the monolayer film. This is not further affected by the

263

subsequent potential scans. In contrast, there is a significant reduction in the

measured charge value in the case of SAM of p-ATP on Au surface after the

potential cycling as can be seen from the Fig. 27D. This indicates the fact that

the monolayer of p-ATP on Au surface undergoes a large structural

organization and also decreased ionic permeability during the process of

potential cycling. This ultimately leads to the formation of a compact and well-

ordered SAM with less number of pinholes and defects.

Figure 27: Cyclic voltammograms of gold oxide stripping analysis in 0.1M

HClO4 for, (A) Bare gold electrode. (B) SAM of thiophenol (TP) on Au

electrode (a), a potential cycled electrode after the SAM formation of TP on Au

(b). (C) SAM of o-aminothiophenol (o-ATP) on Au electrode (a), a potential

cycled electrode after the SAM formation of o-ATP on Au (b). (D) SAM of p-

aminothiophenol (p-ATP) on Au electrode (a), a potential cycled electrode after

the SAM formation of p-ATP on Au (b).

264

We have obtained a surface coverage value of 90% (after potential

cycling) in the cases of SAMs of TP and o-ATP on Au surface and 80% in the

case of p-ATP on Au surface. The surface coverage values calculated from the

gold oxide stripping method are lower than the values obtained using the redox

probes, which can be attributed to the fact that the probe ion in this case is OH−

ions that is very much smaller in size and can have easy access to the electrode

surface through the pinholes and defects when compared to that of other bulkier

ions. In addition to this, it is always possible that potential cycling to large

positive potentials of 1.4V vs. SCE induces some structural damage to the film

which can show low surface coverage determined from the charge values

obtained from the cathodic stripping peak. Infact after repeated cycles in

perchloric acid, the CV is almost similar to that of bare Au electrode, indicating

the complete removal of the monolayer. This confirms that the monolayer is not

electrochemically stable when cycled to high positive potentials.

4.6.3. Formation of mixed SAM with 1-Octanethiol

We have tried to improve the blocking ability of the monolayers of

aromatic thiols on Au surface by forming a mixed SAM with 1-Octanethiol.

Immediately after the adsorption of corresponding aromatic thiols, the

monolayer-coated electrodes were rinsed in ethanol and distilled water and

dipped in neat thiol of 1-Octanethiol for the formation of mixed SAM. After

this procedure, the electrode was rinsed with ethanol, which is followed by

cleaning in distilled water and millipore water for subsequent use in

electrochemical studies.

265

4.6.3.1. Cyclic voltammetry

Figure 28A shows the cyclic voltammograms of mixed SAM modified

electrodes in 10mM potassium ferrocyanide with 1M NaF as a supporting

electrolyte at a potential scan rate of 50mV/s. The mixed SAM modified

electrodes show a good blocking behaviour to the electron transfer reaction.

Figures 28A (a), (b) and (c) show the respective cyclic voltammograms of

mixed SAM coated electrodes using 1-Octanethiol with the monolayers of TP,

o-ATP and p-ATP respectively on Au surface. It can be seen from the figures

that the mixed SAM modified electrodes in the cases of TP and p-ATP show a

better blocking behaviour. But in the case of o-ATP, the mixed SAM coated

electrode exhibits a quasi-reversible behaviour for the redox reaction implying a

poor blocking ability of the monolayer film. This anomalous behaviour will be

discussed later. Interestingly, the blocking property of p-ATP has been

improved to a very large extent after the mixed SAM formation.

Figure 28B shows the comparison of cyclic voltammograms of mixed

SAM coated electrodes in 1mM hexaammineruthenium(III) chloride with 0.1M

LiClO4 as a supporting electrolyte at a potential scan rate of 50mV/s. Figures

28B (a), (b) and (c) show the respective cyclic voltammograms of mixed SAM

modified electrodes using 1-Octanethiol with the monolayers of TP, o-ATP and

p-ATP on Au surface. It can be seen from the figures that the mixed SAMs in

the cases of TP and o-ATP on Au surface do not inhibit the redox reaction of

ruthenium complex, while the mixed SAM in the case of p-ATP on Au surface

exhibits a quasi-reversible behaviour for the electron transfer reaction implying

a rather poor blocking property.

266

Figure 28: (A) Cyclic voltammograms in 10mM potassium ferrocyanide with

1M NaF as a supporting electrolyte at a potential scan rate of 50mV/s of mixed

SAM modified electrodes with 1-Octanethiol for, (a) SAM of TP on Au

electrode, (b) SAM of o-ATP on Au surface and (c) SAM of p-ATP on Au

electrode. (B) Cyclic voltammograms in 1mM hexaammineruthenium(III)

chloride with 0.1M LiClO4 as a supporting electrolyte at a potential scan rate

of 50mV/s of mixed SAM coated electrodes with 1-Octanethiol for, (a) SAM of

TP on Au surface, (b) SAM of o-ATP on Au electrode and (c) SAM of p-ATP on

Au surface.

267

4.6.3.2. Electrochemical impedance spectroscopy

Figure 29A shows the impedance plots (Nyquist plots) of mixed SAM

coated electrodes using 1-Octanethiol with the corresponding monolayers of

TP, o-ATP and p-ATP on Au surface in 10mM potassium ferrocyanide and

10mM potassium ferricyanide with 1M NaF as a supporting electrolyte. Figures

29A (a) and (b) show the respective Nyquist plots of mixed SAM for the cases

of TP and p-ATP on Au surface and the inset shows the corresponding plot for

o-ATP on Au surface. It can be seen from the figure that the mixed SAMs of TP

and p-ATP show that the electron transfer reaction is under charge transfer

control. The Rct value is highest in the case of p-ATP among the three mixed

SAM modified electrodes studied in this work. It can be noted from the inset of

figure that the mixed SAM of o-ATP shows a semicircle at higher frequency

region and a straight line at lower frequency indicating a quasi-reversible

behaviour for the electron transfer reaction, which implies a poor blocking

ability of the monolayer.

Figure 29B shows the Nyquist plots of mixed SAM of different

aromatic thiols and 1-Octanethiol on gold surface in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as a supporting

electrolyte. It can be seen from the inset of the figure that the mixed SAM

coated electrodes in the cases of TP (Fig. 29B (a)) and o-ATP (Fig. 29B (b)) on

Au surface show a straight line almost in the entire frequency range used for the

study implying a diffusion control process for the electron transfer reaction. The

mixed SAM of 1-Octanethiol with p-ATP on Au surface (Fig. 29B) shows a

semicircle at higher frequencies and a straight line at low frequency indicating

the quasi-reversible behaviour for the redox reaction of ruthenium complex.

The charge transfer resistance (Rct) value is higher in the case of p-ATP when

268

compared to other mixed SAM coated electrodes. These results are largely in

conformity with our CV results described in the previous section.

Figure 29: (A) Typical impedance plots in 10mM potassium ferrocyanide and

10mM potassium ferricyanide with 1M NaF as a supporting electrolyte of

mixed SAM modified electrodes with 1-Octanethiol for, (a) SAM of TP on Au

electrode and (b) SAM of p-ATP on Au surface. Inset shows the same plot for

SAM of o-ATP on Au electrode. (B) Impedance plots in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as a supporting

electrolyte of mixed SAM coated electrodes with 1-Octanethiol for the SAM of

p-ATP on Au electrode. Inset shows the similar plots for, (a) SAM of TP on Au

surface and (b) SAM of o-ATP on Au electrode.

269

4.6.4. Formation of mixed SAM with 1,6-Hexanedithiol

We have carried out experiments to improve the blocking property of

the monolayers of aromatic thiols by forming a mixed SAM with an aliphatic

dithiol namely, 1,6-Hexanedithiol. Immediately after the adsorption of the

corresponding aromatic thiols, the monolayer-modified electrodes were rinsed

in ethanol and distilled water and dipped in neat thiol of 1,6-Hexanedithiol for

mixed SAM formation. After this procedure, the electrode was rinsed again

with ethanol followed by cleaning in distilled water and millipore water and

immediately used for the analysis using electrochemical techniques.

4.6.4.1. Cyclic voltammetry

Figure 30A shows the comparison of cyclic voltammograms of mixed

SAM modified electrodes using 1,6-Hexanedithiol in 10mM potassium

ferrocyanide with 1M NaF as a supporting electrolyte at a potential scan rate of

50mV/s. It can be seen from the figures that the mixed SAM modified

electrodes in all the cases of monolayers of TP (Fig. 30A (a)), o-ATP (Fig. 30A

(b)) and p-ATP (Fig. 30A (c)) on Au surface show a good blocking behaviour

to the electron transfer reaction indicating the formation of highly ordered,

well-organized and compact monolayer with less number of pinholes and

defects. Figure 30B shows the CVs of mixed SAM coated electrodes using 1,6-

Hexanedithiol with the corresponding monolayers of TP, o-ATP and p-ATP on

Au surface in 1mM hexaammineruthenium(III) chloride with 0.1M LiClO4 as a

supporting electrolyte at a potential scan rate of 50mV/s. It can be noted from

the figures that the mixed SAM modified electrodes in all the cases of the

monolayers of TP (Fig. 30B (a)), o-ATP (Fig. 30B (b)) and p-ATP (Fig. 30B

(c)) on Au surface show a quasi-reversible behaviour for the redox reaction of

ruthenium complex implying the poor blocking ability of these monolayers.

270

Figure 30: (A) Cyclic voltammograms of mixed SAM modified electrodes with

1,6-Hexanedithiol in 10mM potassium ferrocyanide with 1M NaF as a

supporting electrolyte at a potential scan rate of 50mV/s for, (a) SAM of TP on

Au electrode, (b) SAM of o-ATP on Au electrode and (c) SAM of p-ATP on Au

electrode. (B) Cyclic voltammograms of mixed SAM coated electrodes with 1,6-

Hexanedithiol in 1mM hexaammineruthenium(III) chloride with 0.1M LiClO4

as a supporting electrolyte at a potential scan rate of 50mV/s for, (a) SAM of

TP on Au surface, (b) SAM of o-ATP on Au electrode and (c) SAM of p-ATP on

Au surface.

271

4.6.4.2. Electrochemical impedance spectroscopy

Figure 31A shows the impedance (Nyquist) plots of mixed SAM

modified electrodes using 1,6-Hexanedithiol in 10mM potassium ferrocyanide

and 10mM potassium ferricyanide with 1M NaF as a supporting electrolyte. It

can be noted from the figures that the mixed SAM coated electrodes in all the

cases of the monolayers of TP (Fig. 31A (a)), o-ATP (Fig. 31A (b)) and p-ATP

(Fig. 31A (c)) on Au surface show a semicircle formation almost in the entire

range of frequency used for the study implying the charge transfer control

process for the electron transfer reaction. The mixed SAM modified electrodes

show a good blocking ability to the redox reaction and the charge transfer

resistance (Rct) value increases from TP to o-ATP to p-ATP on Au surface.

Figure 31B shows the Nyquist plots of mixed SAM coated electrodes using 1,6-

Hexanedithiol in 1mM hexaammineruthenium(III) chloride with 0.1M LiClO4

as a supporting electrolyte. It can be seen from the figures that the mixed SAM

modified electrodes in all the cases of the monolayers of TP (Fig. 31B (a)), o-

ATP (Fig. 31B (b)) and p-ATP (Fig. 31B (c)) on Au surface show a depressed

semicircle at higher frequencies and a straight line at lower frequency indicating

a quasi-reversible behaviour for the electron transfer reaction. It can also be

noted that the charge transfer resistance (Rct) value increases in the order p-

ATP>o-ATP>TP SAMs on Au surface. Overall, the blocking ability of these

monolayers has been significantly improved due to the mixed SAM formation.

These results are in conformity with our cyclic voltammetry studies discussed

in the previous section.

272

Figure 31: (A) Impedance plots of mixed SAM modified electrodes with 1,6-

Hexanedithiol in 10mM potassium ferrocyanide and 10mM potassium

ferricyanide with 1M NaF as a supporting electrolyte for, (a) SAM of TP on Au

electrode, (b) SAM of o-ATP on Au surface and (c) SAM of p-ATP on Au

electrode. (B) Typical impedance plots of mixed SAM coated electrodes with

1,6-Hexanedithiol in 1mM hexaammineruthenium(III) chloride with 0.1M

LiClO4 as a supporting electrolyte for, (a) SAM of TP on Au electrode, (b) SAM

of o-ATP on Au surface and (c) SAM of p-ATP on Au electrode.

273

4.6.5. Analysis of impedance data

The impedance data obtained for bare Au electrode, SAM modified

electrodes, potential cycled electrodes (after the SAM formation) and the mixed

SAM coated electrodes were fitted to a suitable equivalent circuit in order to

determine the charge transfer resistance (Rct) value, which is the measure of

blocking ability of the monolayer against the diffusion of redox probes through

the film. The measured charge transfer resistance (Rct) values of SAM coated

electrodes are higher than the corresponding Rct value of the bare Au electrode

due to the inhibition of electron transfer rate by the monolayer film. The

impedance values are fitted to a standard Randle’s equivalent circuit model

comprising of a parallel combination of a constant phase element (CPE)

represented by Q and a faradaic impedance Zf in series with the uncompensated

solution resistance, Ru. The faradaic impedance Zf is a series combination of

charge transfer resistance, Rct and the Warburg impedance, W for the cases of

bare Au electrode, SAM of p-ATP on Au, potential cycled electrodes (after the

SAM formation) and some of the mixed SAM modified electrodes, especially

for the redox reaction of ruthenium complex. For the other cases of monolayer-

coated electrodes, the faradaic impedance Zf consists only of the charge transfer

resistance, Rct. We have determined the Rct values by equivalent circuit fitting

procedure using the impedance plots of the respective bare Au electrode and the

SAM modified electrodes. Table 2 shows the charge transfer resistance (Rct)

values of bare Au electrode and SAM modified electrodes (based on aromatic

thiols) obtained by fitting their corresponding impedance data to a suitable

equivalent circuit. Similarly, table 3 shows the charge transfer resistance values

obtained from the equivalent circuit fitting analysis using impedance plots of

SAMs of o-ATP and p-ATP on Au surface under the acidic and basic

conditions.

274

Table-2

The charge transfer resistance (Rct) values obtained from the impedance plots of

bare Au electrode and various SAM modified electrodes using two different

redox couples as probe molecules.

Table-3

The charge transfer resistance (Rct) values obtained from the impedance plots of

SAMs of o-ATP and p-ATP on Au surface under the acidic and basic

conditions using two different redox couples as probe molecules.

Sample

[Fe(CN)6]3-|4-

Rct (Ω cm2)

[Ru(NH3)6]2+|3+

Rct (Ω cm2)

Bare Au 0.4488 0.216

TP/Au 360.0 137.02

o-ATP/Au 330.8 0.632

p-ATP/Au 2.756 0.707

o-ATP/Au [Rct (Ω cm2)] p-ATP/Au [Rct (Ω cm2)] Redox probe Acidic Basic Acidic Basic [Fe(CN)6]3-|4- 12.438 287.8 3.79 3.432 [Ru(NH3)6]2+|3+ 52.56 2.914 4.044 1.1264

275

It can be seen from the table 2 that the Rct values of SAM modified

electrodes are higher as expected when compared to that of bare Au electrode.

The parameter Rct is a measure of blocking ability of the monolayer to the

electron transfer reactions. Higher the Rct value, better the blocking behaviour

of the SAM towards the redox reactions. It can also be noted from the table 3

that the Rct values of SAMs of o-ATP and p-ATP on Au surface are very much

lower under the acidic and basic conditions implying the poor blocking ability

of SAM towards the redox reaction of ruthenium complex. From the calculated

Rct values, it is clear that the blocking ability of the SAMs follows the order TP

> o-ATP > p-ATP, which is in conformity with our results of cyclic

voltammetry and impedance spectroscopy studies. Table 4 shows the Rct values

of potential cycled electrodes (after SAM formation) obtained from their

corresponding impedance plots. It can be seen from the tables 2 & 4 that the Rct

values of the potential cycled electrodes are very much higher when compared

to the bare Au electrode and the respective SAM modified electrodes. As can be

noted from table 4 that the Rct values are significantly increased after the

potential cycling implying the improvement in the blocking behaviour of the

SAMs of TP and o-ATP on Au surface. Though there is a small increase in the

Rct values in the case of p-ATP on Au surface, the overall blocking ability of

this monolayer film has not improved significantly by potential cycling. As

described earlier, we have carried out the CV and EIS studies on mixed SAMs

of aromatic and aliphatic thiols viz., 1-Octanethiol and 1,6-Hexanedithiol. The

calculated Rct values obtained from the impedance plots are shown in table 5. It

can be seen from the table 5 that the Rct values of both the mixed SAMs of p-

ATP on Au surface are increased to a very large extent indicating the significant

improvement in the blocking behaviour. But in the case of TP SAM coated

electrodes, the electron transfer reaction is not inhibited significantly. Very

276

surprisingly, in the case of o-ATP, the charge transfer resistance (Rct) is very

much decreased implying much poorer blocking property of the monolayer.

Table-4

The charge transfer resistance (Rct) values determined from the impedance plots

of bare Au electrode and various potential cycled electrodes (after the SAM

formation) by equivalent circuit fitting analysis using two different redox

couples as probe molecules. Table-5

The charge transfer resistance (Rct) values obtained from the impedance plots of

mixed SAM modified electrodes by the equivalent circuit fitting analysis using

two different redox couples as probe molecules.

Charge transfer resistance values Rct (Ω cm2)

Sample

[Fe(CN)6]3-|4- [Ru(NH3)6]2+|3+ Bare Au 0.4488 0.216 TP/Au 770.4 345.2 o-ATP/Au 812.4 907.4 p-ATP/Au 5.164 0.780

Charge transfer resistance (Rct) values (Ω cm2)

Mixed SAM with 1-Octanethiol Mixed SAM with 1,6-Hexanedithiol

Sample

[Fe(CN)6]3-|4- [Ru(NH3)6]2+|3+ [Fe(CN)6]3-|4- [Ru(NH3)6]2+|3+

TP/Au 321.2 332.8 415.4 252.2

o-ATP/Au 46.5 50.94 565.4 336.6 p-ATP/Au 1414.2 1097.4 2214.0 392.8

277

From the charge transfer resistance (Rct) values, we have calculated the

surface coverage (θ) of the monolayer on the gold surface using equation (6),

by assuming that the current is due to the presence of pinholes and defects

within the monolayer.

θ = 1 – (Rct/R′ct) (6)

where Rct is the charge transfer resistance of bare Au electrode and R′ct is the

charge transfer resistance of the corresponding SAM modified electrodes. We

have determined a surface coverage value of ~99.9% for the SAMs of TP and

o-ATP on Au surface. In the case of SAM of p-ATP on Au, the surface

coverage value is found to be 84%, which is very low implying the existence of

large number of pinholes and defects in the monolayer. Table 6 shows the

surface coverage values of different potential cycled electrodes and mixed SAM

modified electrodes of aromatic thiols on Au surface obtained from the

respective Rct values. It can be seen from the table 6 that the surface coverage

value is >99.9% in almost all the cases of the monolayer coated electrodes. In

the case of p-ATP on Au surface, the surface coverage value increases

significantly from 91% after the potential cycling to >99.9% after the mixed

SAM formation. This clearly establishes that after potential cycling the

monolayer is structurally organized to form a well-ordered and compact film. In

the case of mixed SAM, the added thiol occupies the pinholes and defects

within the monolayer, thereby increasing the blocking ability of the film with

much less number of pinholes and defects except in the case of SAM of o-ATP

on Au. From these experiments we conclude that both the processes of potential

cycling and the mixed SAM formation significantly increase the blocking

property of the monolayers of TP, o-ATP and p-ATP on Au surface.

278

Table-6

The surface coverage (θ) values obtained for the different potential cycled

electrodes and mixed SAM modified electrodes from the corresponding Rct

values determined using [Fe(CN)6]3-|4- redox couple as a probe molecule.

4.6.6. Pinhole analysis of SAM modified surfaces

Electrochemical impedance spectroscopic studies on the monolayer-

coated electrodes, potential cycled electrodes (after the SAM formation) and

mixed SAM modified electrodes can provide valuable information on the

distribution of pinholes and defects in the monolayer. Finklea et al. [60]

developed a model for the impedance response of a monolayer-coated

electrode, which behaves as an array of microelectrodes. Fawcett and his co-

workers have extensively studied the structural integrity and organization of the

monolayer by pore size analysis using the electrochemical impedance

spectroscopy data [61-63]. Based on the work of Matsuda [64] and Amatore

[65], a model has been developed to fit the faradaic impedance data obtained

for the electron transfer reactions at the SAM modified electrode to understand

the distribution of pinholes and defects within the monolayer. The impedance

Surface coverage (θ) values Mixed SAM modified electrodes

Sample Potential cycled electrodes 1-Octanethiol 1,6-Hexanedithiol

TP/Au 0.9994 0.9986 0.9990

o-ATP/Au 0.9995 0.9904 0.9992 p-ATP/Au 0.9131 0.9997 0.9998

279

expressions have been derived by assuming that the total pinhole area fraction,

(1-θ) is less than 0.1, where θ is the surface coverage of the monolayer on Au

surface. Both the real and imaginary parts of the faradaic impedance values are

plotted as a function of ω-1/2. There are two limiting cases for this model to be

applied. At higher frequencies, the diffusion profiles of each individual

microelectrode constituent of the array are separated, in contrast to the situation

at lower frequencies where there is an overlap of diffusion profiles.

In the case of SAMs of TP, o-ATP and p-ATP on Au surface, the

distribution of pinholes and defects are analyzed using the above-described

model. Figures 32 (a-d) show the real part of the faradaic impedance of

different SAM modified electrodes plotted as a function of ω-1/2. For

comparison, the plot of bare gold electrode is also shown in figure 32 (a).

Figures 32 (b-d) show the plots of real component of faradaic impedance, Z′f

vs. ω-1/2, for the SAMs of TP, o-ATP and p-ATP on Au surface respectively.

Figures 33 (a-c) show the plots of imaginary component of faradaic impedance,

Z′′f vs. ω-1/2 for the above-mentioned electrodes. The faradaic impedance plots

show the features similar to that of an array of microelectrodes. Analysis of

figures 32 and 33 shows that there are two linear domains at high and low

frequencies for the Z′f vs. ω-1/2 plots and a peak formation in the Z′′f vs. ω-1/2

plots corresponding to the frequency of transition between the two linear

domains. According to the model described earlier, this frequency separates the

two time-dependent diffusion profiles for the microelectrodes.

As discussed before, we tried to improve the surface coverage of these

monolayers by subjecting the SAM coated electrodes to potential cycling and

mixed SAM formation with 1-Octanethiol and 1,6-Hexanedithiol. We have

analyzed these results using the above-mentioned model. Both the real (Z′f) and

280

imaginary (Z′′f) components of the faradaic impedance were calculated and

plotted as a function of ω-1/2 (Figures not shown). The faradaic impedance plots

show the features similar to that of an array of microelectrodes. As an example,

we have shown the plots of Z′f vs. ω-1/2 and Z′′f vs. ω-1/2 for the mixed SAM

modified electrodes in the case of SAM of p-ATP on Au surface in the figures

34A and B respectively. We have obtained similar plots for the other cases of

monolayer-coated electrodes (Figures not shown), from which the distribution

of pinholes and defects within the corresponding monolayer are analyzed.

Figure 32: Plots of real part of faradaic impedance (Z′f) vs. ω-1/2 for, (a) Bare

Au electrode, (b) SAM of TP on Au surface, (c) SAM of o-ATP on Au electrode

and (d) SAM of p-ATP on Au surface.

281

Figure 33: Plots of imaginary part of faradaic impedance (Z′′f) as a function of

ω-1/2 for, (a) SAM of TP on Au electrode, (b) SAM of o-ATP on Au surface and

(c) SAM of p-ATP on Au electrode.

282

Figure 34: The faradaic impedance plots of mixed SAM modified electrode for

the monolayer of p-ATP on Au surface with 1,6-Hexanedithiol. (A) Plot of real

part of faradaic impedance (Z′f) as a function of ω-1/2. (B) Plot of imaginary

part of faradaic impedance (Z′′f) vs. ω-1/2.

283

The surface coverage of the monolayer can be determined from the

slope of the Z′f vs. ω-1/2 plot at a higher frequency region, which is given by

[61-63],

m = σ + σ/(1-θ) (5)

and θ = 1 – [σ / (m-σ)] (7)

where m is the slope obtained from the faradaic impedance plots, θ is the

surface coverage of the monolayer and σ the Warburg coefficient, which can be

obtained from the unmodified bare Au electrode.

From the plots, we observed that at lower frequencies the slope of the

curve decreases with increase in ω-1/2. We also find that the Warburg coefficient

(slope m) calculated for the SAM modified electrodes is always higher than that

of the corresponding value of the bare Au electrode. The increase in the

apparent value of the Warburg coefficient of the monolayer-coated electrodes

suggests that the pinholes are distributed in patches over the surface. Using eqn.

(7) we have calculated the surface coverage values of 0.9974, 0.9950, and

0.9200 for the SAMs of TP, o-ATP and p-ATP on Au respectively. The surface

coverage values are very much less compared to the SAMs of aliphatic thiols,

which implies that the aromatic thiols studied in this work do not form a highly

dense, well-organized and compact monolayer when compared to that of long

chain aliphatic thiols. Among the different aromatic thiols studied in this work,

we find that the SAM of p-ATP on Au surface possesses comparatively a

poorer blocking behaviour. The pinholes and defects analysis of SAMs

indicates the existence of a large number of pinholes and defects in the case of

aromatic thiols. We have also calculated the surface coverage values of

different potential cycled electrodes and mixed SAM modified electrodes of the

monolayers of TP, o-ATP and p-ATP on Au surface using eqn. (7) and the

284

corresponding values are given in table 7. It can be seen from the table 7 that

the surface coverage values of these aromatic thiols on Au surface are

significantly increased after the potential cycling and mixed SAM formation,

especially in the case of p-ATP on Au surface. By comparing the tables 6 and 7

it can be noted that these surface coverage values are largely in agreement with

the corresponding surface coverage values determined from the Rct values.

Table-7

The surface coverage (θ) values determined for various potential cycled

electrodes and mixed SAM modified electrodes from the pinholes and defects

analysis using the impedance data obtained for [Fe(CN)6]3-|4- redox couple as a

probe molecule.

Based on the analysis of cyclic voltammetry and electrochemical

impedance spectroscopic studies on SAMs of aromatic thiols on Au surface, we

find that the molecules of TP, o-ATP and p-ATP form a moderately good

blocking monolayer film with pinholes and defects. These monolayers block the

redox reaction of potassium ferro/ferri cyanide redox couple indicating fairly

good blocking behaviour of the SAM. In contrast, the redox reaction of

ruthenium complex is quite facile in all the SAM modified electrodes. Under

Surface coverage (θ) values Mixed SAM modified electrodes

Sample Potential cycled electrodes 1-Octanethiol 1,6-Hexanedithiol

TP/Au 0.9984 0.9975 0.9973

o-ATP/Au 0.9989 0.9887 0.9986 p-ATP/Au 0.9240 0.9989 0.9994

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the acidic conditions, where the SAMs of o-ATP and p-ATP acquire a positive

charge at the electrode surface owing to protonation of –OH group, the redox

reaction of potassium ferro/ferri cyanide complex is facile due to the

electrostatic attraction between the SAM and the redox couple. Surprisingly, the

SAM modified electrodes allow the redox reaction of ruthenium complex under

the acidic condition, where it is expected to show the blocking behaviour due to

the repulsive interactions. This aspect has been explained below. Under the

basic conditions, the SAM modified electrodes show a facile kinetics for

ruthenium redox reaction and somewhat good blocking ability for the potassium

ferro/ferri cyanide reaction. These observations are confirmed quantitatively by

the determination of Rct values, which are shown in the table 2 and 3. From the

studies of electron transfer reactions on the SAM modified electrodes, it is clear

that the monolayers are selective to redox reaction of ruthenium complex.

Similarly, in the case of potential cycled electrodes (after the SAM

formation), the redox reaction of [Fe(CN)6]3-|4- redox couple is inhibited and the

extent of blocking is more after the potential cycling when compared to as

prepared SAM electrodes (Fig. 23 and 25). In contrast, the redox reaction of

ruthenium complex is quite facile even after the potential cycling, although

there is a very small increase in the Rct values (Fig. 24 and 26).

We have carried out experiments in order to evaluate the structural

integrity of the monolayer and to rule out the possibility of any reaction product

blocking the redox reactions of the probe molecules. The experiment involves

carrying out the cyclic voltammetric studies of SAM modified electrodes in the

potential range of –0.1V to 0.5V vs. SCE in 1M NaF aqueous solution. We find

that the CVs do not show any peak formation for the SAMs of aromatic thiols

on Au surface implying that the monolayer is structurally rigid and it does not

undergo any reaction in this potential range where the electron transfer

286

reactions of redox probe molecules are studied. These experiments prove the

fact that the monolayer does not undergo any structural disorganization within

the potential range used for the study.

Usually, the electron transfer reactions on the SAM modified

electrodes usually occur either through the pinholes and defects or by a

tunneling process. In the case of aromatic thiols, the electron transfer can be

further facilitated by the delocalized π-electrons present in the aromatic phenyl

ring because of extended conjugation. It may be pointed out that the sizes of

ferrocyanide (6Å) and ruthenium (6.4Å) complexes are almost comparable [19]

and if ruthenium redox reaction is assumed to occur by access to the electrode

surface through the pinholes and defects present in the monolayer, then the

redox reaction of ferrocyanide is also expected to show facile kinetics.

Obviously, it is not so and therefore it is felt that the Ru(III) redox reaction may

take place by a tunneling process through the monolayer. It is well known that

the redox reaction of hexaammineruthenium(III) chloride follows an outer

sphere electron transfer reaction [35], where the physical access to the electrode

surface is not a pre-requisite for the electron transfer to occur since the process

is facilitated by a tunneling mechanism. This is in contrast to ferrocyanide

redox reaction, which is an inner sphere electron transfer reaction, where the

access of redox species to the electrode surface is necessary for the reaction to

occur [36]. These results are in conformity with our studies on SAMs of 2-

naphathelenethiol on Au surface (discussed in the previous section), in which

the aromatic ring with highly delocalized π-electrons mediate the electron

transfer between the SAM and the redox species. We believe a similar process

can occur in the cases of SAMs of TP, o-ATP and p-ATP on Au surface.

The process of potential cycling has introduced significant structural

organization on the monolayers, resulting in the formation of highly compact,

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well-ordered monolayers with less number of pinholes and defects. It can also

be seen from our experiments that although the potential cycling has generally

improved the blocking ability of the monolayers of aromatic thiols on Au

surface, we do not find significant improvement in the case of p-ATP SAM. We

have studied mixed SAM formation with 1-Octanethiol and 1,6-Hexanedithiol

(we have chosen these thiols because of comparable chain lengths to that of the

aromatic thiols) essentially to improve the blocking behaviour of the monolayer

of p-ATP on Au electrode. The adsorption energy of aliphatic thiols to the gold

surface is lower in comparison to the energy required for the replacement of

existing aromatic thiols. The added aliphatic thiol and dithiol molecules

therefore fill up the pinholes, defects and domain boundaries present within the

monolayers, resulting in the formation of highly organized, well ordered,

compact monolayer leading to a significant improvement in the blocking

efficiency of these SAMs. It can be noted from the cyclic voltammetry and

impedance spectroscopic studies that the mixed SAM coated electrodes show

significant improvement in the blocking behaviour when compared to simple

SAM modified electrodes, especially in the case of p-ATP on Au surface. Even

in the cases of mixed SAM modified electrodes, the redox reaction of

ruthenium complex shows the quasi-reversible behaviour indicating a tunneling

mechanism for the electron transfer process. On the other hand, the redox

reaction of potassium ferrocyanide is fully inhibited. It can be noted that the

mixed SAM of o-ATP with 1-Octanethiol show a quasi-reversible behaviour for

the redox reaction implying a rather poor blocking ability of the monolayer. In

the case of SAM of o-ATP on Au surface, the nitrogen atom from the amino

group is also known to bind with the gold surface through the lone pair of

electrons [88]. The intercalation of alkanethiol molecules within the space is

inhibited by the amino group of o-ATP and not in the case of p-ATP, as it is

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vertically oriented. The blocking ability of the monolayer of p-ATP on Au

surface is significantly improved after the mixed SAM formation, which is

evidenced by the large increase in the Rct values and the surface coverage (θ)

values, as can be seen from the tables 5, 6 and 7. On the whole, the blocking

ability of the monolayers of TP and p-ATP on Au surface is improved to a large

extent by the process of potential cycling and mixed SAM formation. In the

case of o-ATP, only the process of potential cycling improves the blocking

ability of the monolayer and not the mixed SAM formation.

Based on the experimental results, it is proposed that the redox reaction

of [Fe(CN)6]3-|4- occurs mainly through the pinholes and defects present in the

monolayers. On the other hand, the redox reaction of [Ru(NH3)6]2+|3+ occurs by

a tunneling through bond mechanism where the SAMs act as a mediator for the

electron transfer between Au surface and the redox couple. In these cases, the

process of tunneling is facilitated not only by the presence of highly delocalized

π-electrons in the aromatic phenyl ring but also by the smaller length of the

aromatic molecules used for the monolayer formation and its characterization.

We think that our results on the study of electron transfer reactions are

important in biological systems involving proteins and DNA, where the

structural units contain mainly aromatic core that play a vital role in the electron

transfer mechanisms. The SAM modified electrodes are the ideal candidates to

mimic the biological systems in order to understand the various physiological

and chemical processes occurring in these systems.

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III. SAMs OF ALKOXYCYANOBIPHEYL THIOLS ON Au SURFACE

4.7. Introduction

Similar to aromatic thiols, we have also studied the monolayer

formation and characterization of alkoxycyanobiphenyl thiols having different

alkyl chain lengths on gold surface using dichloromethane as a solvent. The

special feature of these alkoxycyanobiphenyl thiols is that it contains both the

aliphatic and aromatic parts in its structure. These molecules show a nematic

liquid crystalline phase in the bulk sample. The fundamental studies on

interfacial electron transfer and electrochemical processes at the SAM modified

electrode | solution interface are attaining a great deal of interest and widely

reported in literature [37,41,101]. The blocking ability of the monolayer-coated

electrode to the electron transfer process is usually evaluated by studying the

redox reactions using potassium ferro/ferri cyanide and

hexaammineruthenium(III) chloride complexes as redox probes [61-63,76].

Some ruthenium complexes in solutions have also been used as redox probes in

many chemical, biological and photochemical sensors [102-107].

Apart from commonly studied SAMs of aliphatic thiols, there is also a

great deal of interest on the monolayers of aromatic thiols [79,81,84,86] in

recent times. SAMs of aromatic thiols are of interest owing to their higher

rigidity and the presence of delocalized π-electrons in the aromatic ring. Among

several aromatic thiols reported in literature, there are some scattered reports on

SAM of biphenyl thiol [108-110]. Cyganik et al. reported the scanning

tunneling microscopic observation of the influence of spacer chain on the

molecular packing of SAMs of ω-biphenylalkanethiols on Au (111) [108].

Long et al. studied the effect of odd-even number of alkyl chains on SAMs of

biphenyl-based thiols using cyclic voltammetry [109].

290

Reports on self-assembled monolayer of molecules, which show the

liquid crystalline phase behaviour in bulk are also very rare. In literature, both

discotic and calamitic (types of liquid crystalline phases) molecules have been

shown to form highly ordered SAMs on gold leading to many interesting

properties and phenomena [111-117]. A number of terminally substituted

alkoxycyanobiphenyl compounds such as bromo-, hydroxy-, amino-, carboxy-,

epoxy- and olefine- terminated cyanobiphenyls are already known. However,

we find that the terminal thiol functionalized cyanobiphenyls have not yet been

explored. Recently, Kumar et al. reported the synthesis of a number of terminal

thiol-substituted alkoxycyanobiphenyls [118] that have been used for the

monolayer preparation and characterization in the present work.

In this section, we report our studies on self-assembled monolayer

films of some alkoxycyanobiphenyl molecules functionalized with thiols. The

liquid crystalline phase behaviour of these molecules is confirmed by polarizing

light microscopy. Electrochemical techniques such as cyclic voltammetry (CV)

and electrochemical impedance spectroscopy (EIS) were used for the evaluation

of barrier property of the SAM-modified electrodes using potassium

ferrocyanide and hexaammineruthenium(III) chloride complexes as redox

probes. Impedance spectroscopy data were used for the calculation of surface

coverage and other kinetic parameters for the SAM modified electrodes.

4.8. Experimental section

4.8.1. Chemicals

Dichloromethane (Spectrochem), potassium ferrocyanide (Loba),

potassium ferricyanide (Qualigens), hexaammineruthenium(III) chloride (Alfa

Aesar), sodium fluoride (Qualigens) and lithium perchlorate (Acros Organics)

were used in this study as received. Alkoxycyanobiphenyl thiol compounds of

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different chain lengths (C5(a), C8(b) and C10(c)) were synthesized by Kumar et

al. [118] and the synthetic scheme is described elsewhere [119]. All the

chemical reagents used in this work were analytical grade (AR) reagents.

Millipore water having a resistivity of 18 MΩ cm was used to prepare the

aqueous solutions. The structure of the alkoxycyanobiphenyl thiol compounds

studied in this work is shown in the following figure.

4.8.2. Sample preparation

Gold sample of purity 99.99% was obtained from Arora Mathey, India.

Evaporated gold (~100 nm thickness) on glass with chromium underlayers (~2-

5 nm thickness) was used for the monolayer formation and its characterization

using electrochemical techniques. The substrate was heated to 3500C during

gold evaporation under a vacuum pressure of 2 X 10-5 mbar, a process that

normally yields a very smooth gold substrate with predominantly Au (111)

orientation. The evaporated gold samples were used as strips for SAM

formation and its analysis.

C5 (a)

C8 (b)

C10 (c)

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4.8.3. Preparation of SAMs of alkoxycyanobiphenyl thiols on Au

Self-assembled monolayers (SAMs) of the above-mentioned

alkoxycyanobiphenyl thiols of different chain length (C5(a), C8(b) and C10(c)),

have been formed on gold and characterized using electrochemical techniques.

The evaporated Au samples were used as strips for the monolayer preparation

and its characterization. Before SAM formation, the gold strips were pretreated

with “piranha” solution, which is a mixture of 30% H2O2 and conc. H2SO4 in

1:3 ratio. The monolayers were prepared by keeping the Au strips in 1mM thiol

solution in dichloromethane for about 1 hour and 15 hours. After the adsorption

of thiol, the Au coated electrode was rinsed with dichloromethane; thoroughly

cleaned using distilled water and finally with millipore water and used for the

analysis immediately. For comparison the self-assembled monolayers of

decanethiol and hexadecanethiol on Au were also prepared using a similar

procedure.

4.8.4. Electrochemical characterization of SAMs on Au surface

A conventional three-electrode electrochemical cell with large surface

area Pt foil as a counter electrode, saturated calomel electrode (SCE) as a

reference electrode and SAM modified Au strips as working electrode was used

for the electrochemical characterization of SAMs. The barrier property of the

monolayer has been evaluated by studying the electron transfer reaction on the

modified surfaces using two different redox probes namely potassium

ferrocyanide (negative redox probe) and hexaammineruthenium(III) chloride

(positive redox probe). Cyclic voltammetric measurements were carried out in

10mM potassium ferrocyanide in 1M NaF at a potential range of –100mV to

500mV vs. SCE and 1mM hexaammineruthenium(III) chloride in 0.1M LiClO4

at a potential range of –400mV to 100mV vs. SCE. The impedance

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measurements were carried out using an ac signal of 10mV amplitude at a

formal potential of the redox couple at a wide frequency range of 100kHz to

0.1Hz. The electrolytic solution contained equal concentrations of both the

oxidized and reduced forms of the redox couple namely, 10mM potassium

ferrocyanide and 10mM potassium ferricyanide in 1M NaF. From the

impedance data, the charge transfer resistance (Rct) was determined using the

equivalent circuit fitting analysis. From the Rct values, the surface coverage (θ)

of the monolayer on Au surface and the rate constant of the electron transfer

reaction on the SAM modified electrodes were calculated.

4.8.5. Instrumentation

The cyclic voltammetric studies were performed using an EG&G

potentiostat (model 263A) interfaced to a PC through a GPIB card (National

Instruments). For electrochemical impedance spectroscopy studies the

potentiostat is used along with an EG&G 5210 lock-in-amplifier controlled by

Power Sine software (EG&G). The equivalent circuit fitting of the impedance

data has been carried out using Zsimpwin software (EG&G) developed on the

basis of Boukamp’s model.

4.9. Results and discussion

4.9.1. Cyclic voltammetry

Cyclic voltammetry is an important technique to evaluate the blocking

property of the monolayer-coated electrodes using diffusion controlled redox

couples as probes. Figure 35A shows the cyclic voltammograms of bare Au and

SAM modified Au electrodes in 10mM potassium ferrocyanide with 1M NaF as

the supporting electrolyte at a potential scan rate of 50mV/s. It can be seen from

the figure that the bare Au electrode (Fig. 35A (a)) shows a reversible

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voltammogram for the redox couple indicating that the electron transfer

reaction is completely diffusion controlled. In contrast, the absence of any peak

formation in the CVs of the monolayer modified electrodes shows that the

redox reaction is inhibited. It can also be seen that in the case of C5 thiol (Fig.

35A (b)), the CV exhibits a rather imperfect blocking behaviour. In the case of

C8 and C10 thiols (Fig. 35A (c) and 35A (d)) the CVs indicate a good blocking

behaviour for the electron transfer reaction, which means that a highly ordered,

compact monolayer of alkoxycyanobiphenyl thiol is formed on the Au surface.

The CVs also exhibit the microelectrode array characteristics [98-100]. The

microelectrode array behaviour obtained in the case of C8 and C10 thiols arises

due to the access of ions through the pinholes and pores present in the SAM,

which facilitates the radial diffusion of the redox species in contrast to the linear

diffusion, observed in the case of bare Au surface.

Figure 35B shows the cyclic voltammograms of bare Au and

monolayer-modified electrodes in 1mM hexaammineruthenium(III) chloride

with 0.1M LiClO4 as the supporting electrolyte at a potential scan rate of

50mV/s. It can be seen from the figure that the bare Au electrode (Fig. 35B (a))

shows a reversible behaviour implying that the redox reaction is under diffusion

control. In contrast, the SAM modified electrodes in the case of C8 (Fig. 35B

(c)) and C10 thiols (Fig. 35B (d)) show a good blocking behaviour. The CVs

exhibit characteristics that are typical of an array of microelectrodes [98-100].

On the other hand, the ruthenium redox reaction through the SAM of C5 thiol

(Fig. 35B (b)) shows a quasi-reversible behaviour with peak current values

close to that of the bare Au electrode. It is worth recalling that the redox

reaction of [Fe(CN)6]3-|4- is blocked in the case of C5 thiol.

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Figure 35: (A) Cyclic voltammograms in 10mM potassium ferrocyanide with

1M NaF as supporting electrolyte at a potential scan rate of 50mV/s for (a)

bare Au electrode, (b), (c) and (d) are SAMs of alkoxycyanobiphenyl thiols with

different alkyl chain lengths of C5, C8 and C10 on Au surface respectively. (B)

Cyclic voltammograms in 1mM hexaammineruthenium(III) chloride with 0.1M

LiClO4 as supporting electrolyte at a potential scan rate of 50mV/s for (a) bare

Au electrode, (b), (c) and (d) are SAMs of alkoxycyanobiphenyl thiols with

various alkyl chain lengths of C5, C8 and C10 on Au surface respectively.

296

In order to understand the mechanism of electron transfer process of

hexaammineruthenium(III) chloride through the SAM of C5 thiol on Au

electrode, we have carried out similar studies using aliphatic thiols with methyl

group at the terminal position namely decanethiol and hexadecanethiol, which

have chain lengths comparable to that of C5 and C8 alkoxycyanobiphenyl thiols.

Though we have carried out the experiments for both the aliphatic thiols, we

discuss the results obtained on the SAM of decanethiol on Au, which has a

shorter chain length and comparable to C5 thiol. The monolayer was prepared

by keeping the Au strips in 1mM decanethiol solution in dichloromethane for

about 15 hours and the results are compared with the monolayer of C5 thiol on

Au obtained using a similar procedure. After the adsorption of thiol, the SAM

coated Au electrodes were rinsed with dichloromethane, thoroughly cleaned

using distilled water and finally with millipore water and used for the analysis

immediately.

Figure 36A shows the cyclic voltammograms of bare Au and SAM

modified Au electrodes in 10mM potassium ferrocyanide with 1M NaF as the

supporting electrolyte at a potential scan rate of 50mV/s. It can be seen from the

figure that the bare Au electrode (Fig. 36A (a)) shows a reversible

voltammogram for the redox couple indicating the diffusion-controlled process

for the electron transfer reaction on the bare Au surface. On the other hand,

CVs of SAM modified electrodes show that the redox reaction is completely

inhibited. Figures 36A (b) and 36A (c) show the CVs of SAMs of C5 thiol and

decanethiol on Au coated electrodes. It can be noted that the CVs show the

characteristics of microelectrode array behaviour, indicating the formation of a

highly ordered, compact monolayer on the Au surface.

Figure 36B shows the comparison of cyclic voltammograms of bare

Au and SAM modified electrodes in 1mM hexaammineruthenium(III) chloride

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with 0.1M LiClO4 as the supporting electrolyte at a potential scan rate of

50mV/s. It can be seen from the figure that the bare Au electrode (Fig. 36B (a))

shows a reversible behaviour for the electron transfer reaction implying the

process is under diffusion control. In contrast, the CV of SAM of decanethiol

on Au (Fig. 36B (c)) shows a very good blocking behaviour indicating the

characteristics of an array of microelectrodes. On the other hand, the CV of

SAM of C5 thiol on Au (Fig. 36B (b)) shows a quasi-reversible behaviour with

a large peak-to-peak separation value and the peak current values are close to

that of bare Au electrode. It can be noted that the Ru(III) redox reaction is

almost completely inhibited in the case of decanethiol in contrast to that of

SAM of C5 thiol although the chain lengths of these two thiols are comparable.

4.9.2. Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy is a powerful tool to

determine the kinetic parameters and the surface coverage with the structural

integrity of the monolayer on Au surface by evaluating the presence of pores

and pinholes using redox couple as the probe molecule.

4.9.2.1. Potassium Ferrocyanide – Ferricyanide redox reaction

Figure 37A shows the impedance plots (Nyquist plots) of the SAM

modified electrodes based on C8 and C10 (Fig. 37A (a) and (b)) thiols in equal

concentrations of potassium ferro/ferri cyanide with NaF as the supporting

electrolyte. Figure 37B shows the same plot for SAM of C5 thiol-modified

electrode and the inset shows for bare Au electrode. It can be seen from the

inset of figure 37B that the bare Au shows a low frequency straight line with a

very small semicircle at high frequency region indicating a diffusion-controlled

process for the redox couple on bare Au surface.

298

Figure 36: (A) Cyclic voltammograms in 10mM potassium ferrocyanide with

1M NaF as supporting electrolyte at a potential scan rate of 50mV/s for (a)

bare Au electrode, (b) and (c) are the respective SAMs of C5

alkoxycyanobiphenyl thiol and decanethiol on Au obtained by keeping the Au

strips in 1mM thiol solution for about 15 hours. (B) Cyclic voltammograms in

1mM hexaammineruthenium(III) chloride with 0.1M LiClO4 as supporting

electrolyte at a potential scan rate of 50mV/s for (a) bare Au electrode, (b) and

(c) are the respective SAMs of C5 thiol and decanethiol on Au obtained by

keeping the Au strips in 1mM thiol solution for about 15 hours.

299

Figure 37: Impedance plots in equal concentrations of both potassium

ferrocyanide and potassium ferricyanide solution with NaF as the supporting

electrolyte for, (A) SAMs of C8 (a) and C10 (b) alkoxycyanobiphenyl thiols on

Au surface. (B) SAM of C5 alkoxycyanobiphenyl thiol on Au electrode. (C)

SAMs of C5 thiol (a) and decanethiol (b) on Au surface (15 hours). Inset shows

the same plot for the bare Au electrode.

300

In the case of C5, there is a semicircle at high frequency region and a

straight line at low frequency region, again indicating a diffusion controlled

process implying that the SAM of C5 thiol has a somewhat poor blocking

ability. However the impedance plots of C8 and C10 thiols (Figures 37A (a) and

(b)) show large semicircles in the entire range of frequency, characteristic of

complete charge transfer control implying a perfect blocking behaviour.

Figure 37C shows the Nyquist plots of SAM modified gold electrodes

obtained by keeping the Au strips in thiol solution for about 15 hours to

improve the monolayer characteristics. Figures 37C (a) and 37C (b) show the

respective Nyquist plots of SAM of C5 thiol and decanethiol on Au in equal

concentrations of potassium ferro/ferri cyanide with NaF as the supporting

electrolyte. It can be seen from the figure that the SAM of decanethiol on Au

(Fig. 37C (b)) shows a large semicircle formation in the entire range of

frequency indicating the complete charge transfer control for the redox reaction.

In the case of C5 thiol, the formation of depressed semicircle implies that the

electron transfer process is under charge transfer control with a poor blocking

ability of the monolayer.

4.9.2.2. Hexaammineruthenium(II) –ruthenium(III) redox reaction

Figure 38A shows the Nyquist plots of the SAM modified electrodes

based on C8 and C10 (Fig. 38A (a) and (b)) thiol molecules in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as the supporting

electrolyte. Figure 38B shows a similar plot for the SAM of C5 thiol-modified

electrode and the inset shows the same plot for bare Au electrode. The

impedance plot of bare Au electrode (Inset of Fig. 38B) exhibits a low

frequency straight line with a very small semicircle formation at high frequency

region implying the diffusion controlled process for the Ru(III) electron transfer

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reaction. Similarly, the SAM of C5 thiol on Au (Fig. 38B) shows that the

Ru(III) reaction is quite facile. The small semicircle at high frequency region

implies that the electron transfer reaction is quasi-reversible. Such a behaviour

indicates that the SAM shows a very poor blocking ability. In contrast, the

SAMs based on C8 and C10 (Fig. 38A (a) and (b)) thiol molecules show the

formation of larger semicircles (almost in the entire range of frequency) with a

straight line at very low frequency implying that they form a better blocking

monolayer film.

Figure 38C shows the Nyquist plots of monolayer modified electrodes

on Au obtained by keeping Au strips in thiol solution for about 15 hours. Figure

38C shows the Nyquist plot of SAM of decanethiol on Au in 1mM

hexaammineruthenium(III) chloride with 0.1M LiClO4 as the supporting

electrolyte. Inset shows a similar plot for the SAM of C5 thiol on Au surface. It

can be seen from the inset of figure that the SAM of C5 thiol shows a semicircle

at high frequency region and a straight line at low frequency region, indicating

the facile nature of the ruthenium redox reaction. The plot obtained is typical

characteristics of a quasi-reversible redox reaction implying the poor blocking

ability of the monolayer. The formation of a large semicircle in the entire range

of frequency in the case of SAM of decanethiol on Au surface indicates the

complete charge transfer control for the redox reaction with a very good

blocking behaviour. These results are in conformity with our observations using

cyclic voltammetric studies discussed earlier.

4.9.3. Analysis of impedance data

The impedance values are fitted to a standard Randle’s equivalent

circuit comprising of a parallel combination of a constant phase element (CPE)

represented by Q and a faradaic impedance Zf in series with the uncompensated

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solution resistance, Ru for the cases of bare Au surface and SAM of C5 thiol

modified electrode.

Figure 38: Impedance plots in 1mM hexaammineruthenium(III) chloride with

0.1M LiClO4 as supporting electrolyte for, (A) SAMs of C8 (a) and C10 (b)

alkoxycyanobiphenyl thiols on Au surface. (B) SAM of C5 thiol on Au electrode.

Inset shows the same impedance plot for the bare Au electrode. (C) SAM of

decanethiol on Au surface (15 hours). Inset shows the same impedance plot for

the SAM of C5 thiol on Au electrode (15 hours).

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The faradaic impedance, Zf is a series combination of charge transfer

resistance, Rct and the Warburg impedance, W. For C8 and C10 thiols modified

Au electrodes, the Zf consists only of charge transfer resistance, Rct. Table 8

shows the charge transfer resistance values (Rct) of bare gold and SAM

modified electrodes obtained from the impedance plots. It can be seen from the

table that the Rct values of SAM modified electrodes, as expected are very much

higher when compared to bare Au electrode due to the inhibition of electron

transfer rate by the presence of monolayer on the electrode surface. From the

Rct values, we can calculate the surface coverage (θ) of the monolayer on the

gold electrode using equation (6), by assuming that the current is due to the

presence of defects within the monolayer.

θ = 1- (Rct/R′ct) (6)

where Rct is the charge transfer resistance of bare Au electrode and R′ct is the

charge transfer resistance of the corresponding SAM modified electrodes.

Table-8

The charge transfer resistance (Rct) values obtained from the impedance plots of

the different electrodes using two different redox couples as probe molecules.

Charge transfer resistance (Rct)

values (Ω cm2)

Sample

[Fe(CN)6]3-|4- [Ru(NH3)6]2+|3+

Bare Au 0.5535 1.26

C5 thiol/Au 220.6 65.5

C8 thiol/Au 6636.6 952.8

C10 thiol/Au 6953.75 1362

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The surface coverage values determined from the Rct for all the

alkoxycyanobiphenyl thiols used in this work are presented in table 9. From the

calculated Rct values it is clear that the higher alkyl chain containing

alkoxycyanobiphenyl thiols form a better blocking monolayer and the extent of

blocking follows the order C10 > C8 > C5, which is in conformity with our CV

studies.

Using the Rct values obtained from the impedance plots, we have

determined the rate constant value of [Fe(CN)6]3-|4- for the SAM modified

electrodes. The monolayer, acting as an array of microelectrodes, provides a

barrier for electron transfer reaction leading to an expected decrease in rate

constant values. The Rct can be expressed as follows,

Rct = RT/nFIo (8)

and Io = nFAkC (9)

Substituting eqn. (9) in (8), we get,

Rct = RT/n2F2AkC (10)

From eqn. (10), for a one electron first order reaction with C0=Cr=C and for unit

geometric area, the apparent rate constant can be given as follows,

kapp = RT/F2RctC (11)

The real rate constant ko can be expressed as,

ko = kapp/(1-θ) (12)

where R is the gas constant, T the temperature, F the Faraday’s constant, n the

number of electrons, Io the exchange current density, A the area of electrode, C

the concentration of the redox couple, Rct the charge transfer resistance, θ the

surface coverage, kapp and ko the apparent and the real rate constants

respectively.

Using equations (11) and (12), we have calculated the real and

apparent rate constants for bare Au and SAM modified electrodes. It was found

305

that the apparent rate constant values of monolayer coated electrodes are almost

four orders of magnitude lower when compared to the rate constant of bare Au

electrode. The surface coverage and rate constants obtained from Rct values of

monolayer modified electrodes and the bare Au electrode using [Fe(CN)6]3-|4-

redox couple as probe are shown in table 9.

Table-9

The surface coverage, the apparent and real rate constant values calculated for

[Fe(CN)6]3-|4- redox couple using Rct values obtained from the impedance plots

of different SAM modified electrodes.

It may be pointed out that the sizes of ferrocyanide (6Å) and ruthenium

(6.4Å) complexes are almost comparable and if ruthenium redox reaction is

assumed to occur by access of the redox species to the electrode surface through

the pinholes and defects present in the monolayer, then the reaction of

ferrocyanide should also exhibit a facile kinetics. Since it is not so, it is felt that

the Ru(III) redox reaction does not take place by access of the redox species to

the electrode surface but by a tunneling process through the bond. The C5

molecule with a relatively short alkyl chain length of 7.7Å of the methylene

Sample Surface coverage (θ)

Apparent rate constant kapp (cm/s)

Real rate constant ko (cm/s)

Bare Au ---- ------ 0.4824 C5 thiol/Au 0.9975 1.206 × 10-3 0.4807 C8 thiol/Au 0.9999 0.400 × 10-4 0.4000 C10 thiol/Au 0.9999 0.383 × 10-4 0.3830

306

units (as shown in Fig. 41) and the delocalized π-electrons of the aromatic ring

in series can bridge the redox species and the gold surface effectively to

facilitate the electron transfer process. It is well known that the redox reaction

of hexaammineruthenium(III) chloride follows the outer sphere electron

transfer reaction. In this case physical access to the electrode surface is not a

pre-requisite for the electron transfer to occur since the process is facilitated by

tunneling mechanism. This is in contrast to ferrocyanide redox reaction, which

is an inner sphere electron transfer reaction where the access of redox species to

the electrode surface is necessary for the electron transfer reaction to occur [36].

However in the case of C8 (11.6Å) and C10 (14.1Å) thiols, with longer chain

lengths, the reaction rate decays exponentially with distance with a decay length

β of 0.8 per Å units [119-121]. In this case the access of redox species to the

electrode surface is controlled mainly through the pinholes and pores present in

the monolayer.

From the impedance plots of Fig. 37C, we have obtained the surface

coverage values of 0.9997 and 0.9999 for the SAMs of C5 thiol and decanethiol

on Au surface. Obviously, forming the SAM for longer time of about 15 hours

has the effect of reducing the pinholes and defects significantly. It can be seen

from the Fig. 36B that the ruthenium redox reaction is quite facile in the case of

SAM of C5 thiol in contrast to the SAM of decanethiol on Au, although their

chain lengths and surface coverage values are comparable. It can also be

recalled that both these thiols are equally blocking the ferrocyanide redox

reaction. The facile nature of Ru(III) redox reaction therefore suggests that in

the case of C5 thiol the tunneling through the delocalized π-electrons of the

aromatic ring is a dominating mechanism for the electron transfer process. The

307

very low density of pinholes and defects cannot account for the fast electron

transfer kinetics in the case of ruthenium complex.

4.9.4. Pinhole analysis

The study of electrochemical impedance spectroscopy on SAM

modified electrodes provides a valuable information on the distribution of

pinholes and defects in the monolayer. Finklea et al. [60] developed a model for

the impedance response of a SAM modified electrode, which behaves as an

array of microelectrodes. Based on the work of Matsuda [64] and Amatore [65],

a model has been developed to fit the faradaic impedance data obtained for the

electron transfer reactions at the monolayer-modified electrode, to understand

the distribution of pinholes in the monolayer. The impedance expressions have

been derived by assuming that the total pinhole area fraction, (1-θ) is less than

0.1, where θ is the surface coverage of the monolayer. Both the real and

imaginary parts of the faradaic impedance values are plotted as a function of ω-

1/2. At higher frequencies the diffusion profiles of each individual

microelectrode constituent of the array is separated in contrast to the situation at

lower frequencies where there is an overlap.

In the case of SAM of alkoxycyanobiphenyl thiols on Au surface, the

presence of pinholes and defects are analyzed using the above described model.

Figures 39 (a-d) show the real part of the faradaic impedance of different SAM

modified electrodes plotted as a function of ω-1/2. For comparison the plot of

bare gold electrode is also shown in Fig. 39(a). Figures 39 (b-d) show the plots

of real component of faradaic impedance Z′f vs. ω-1/2 for the monolayer coated

electrodes of alkoxycyanobiphenyl thiols with different alkyl chain lengths of

C5, C8 and C10 respectively. Figures 40 (a-c) show the plots of imaginary

308

component of the faradaic impedance Z′′f vs. ω-1/2 for the above-mentioned

electrodes. The faradaic impedance plots have features similar to that of an

array of microelectrodes. Analysis of figures 39 and 40 shows that there are two

linear domains at high and low frequencies for the Z′f vs. ω-1/2 plots and a peak

formation in the Z′′f vs. ω-1/2 plots corresponding to the frequency of transition

between the two linear domains. According to the model described earlier, this

frequency separates the two time dependent diffusion profiles for the

microelectrodes.

The surface coverage of the monolayer can be determined from the

slope of the Z′f vs. ω-1/2 plot at high frequency region and it is given by,

θ = 1- [σ/(m-σ)] (7)

where m is the slope obtained from the faradaic impedance plots, θ the surface

coverage of the monolayer and σ the Warburg coefficient, which can be

obtained from the unmodified bare gold electrode.

It can be seen from Fig. 39 that at lower frequency the slope of the

curve is decreased as ω-1/2 is increased. We also find the Warburg coefficient

(slope m) calculated for the SAM modified electrodes is always greater than

that obtained from the bare gold electrode. The increase in the apparent value of

the Warburg coefficient for monolayer-coated electrodes suggests that the

pinholes are distributed in patches over the surface. Using eqn. (7) we have

calculated the surface coverage values of 0.9978, 0.9994 and 0.9996 for the

SAMs of C5, C8 and C10 thiols on Au respectively and these values are in good

agreement with the surface coverage values determined from the Rct values

shown in table 9.

From the studies of cyclic voltammetry and electrochemical impedance

spectroscopy on SAMs of alkoxycyanobiphenyl thiols on Au surface, we find

309

that these liquid crystalline molecules form a highly dense and well-packed

compact monolayer. These monolayers block the redox reaction of potassium

ferro/ferri cyanide couple indicating the good barrier property of the SAMs on

Au surface. In contrast, the redox reaction [Ru(NH3)6]2+|3+ is quite facile when

the alkyl chain length is shorter (C5) whereas it is inhibited when the alkyl chain

length is longer (C8 & C10).

Figure 39: Plots of real part of faradaic impedance (Z′f) vs. ω-1/2 for, (a) Bare

Au electrode, (b) SAM of C5 thiol on Au electrode, (c) SAM of C8 thiol on Au

surface and (d) SAM of C10 thiol on Au electrode.

310

Figure 40: Plots of imaginary part of faradaic impedance (Z′′f) as a function of

ω-1/2 for, (a) SAM of C5 thiol on Au electrode, (b) SAM of C8 thiol on Au surface

and (c) SAM of C10 thiol on Au electrode.

311

Figure 41 shows the proposed mechanism for the Ru(III) electron

transfer reaction on the SAM modified electrodes. In the case of C5 thiol, the

SAM provides tunneling pathway for Ru(III) redox reaction by mediating the

electron transfer between Au surface and the redox couple. Our results are

significant in studies involving electron transfer through molecules containing

aromatic core and aliphatic chains in their structure. In these cases the aromatic

core with delocalized electrons facilitates the charge transfer while the short

methylene (aliphatic) chains provide through bond tunneling pathway. Such

systems have interesting applications in the study of bridge mediated electron

transfer reactions and in the study of biological sensors and molecular

electronics.

Figure 41: Schematic representation of proposed mechanism for electron

transfer reaction on SAM modified electrodes using [Ru(NH3)6]2+|3+ as redox

probe. SAMs formed by liquid crystalline molecules of thiol terminated

alkoxycyanobiphenyls mediate electron transfer between Au electrode and

[Ru(NH3)6]2+|3+ when the alkyl chain length is shorter (C5) and inhibit the

process when the alkyl chain length is longer (C8 & C10).

312

4.10. Conclusions

In this chapter, we have described the monolayer formation and its

characterization of some aromatic thiols and alkoxycyanobiphenyl thiols on Au

surface obtained using organic solvents as an adsorbing medium. We have

studied the monolayer formation of 2-naphthalenethiol, thiophenol, o-

aminothiophenol, p-aminothiophenol and alkoxycyanobiphenyl thiols having

different alkyl chain lengths of C5, C8 and C10 on Au surface. The barrier

property, insulating property and blocking ability of these monolayers were

evaluated using electrochemical techniques such as cyclic voltammetry,

electrochemical impedance spectroscopy and gold oxide stripping analysis. We

have used two important redox couples namely, [Fe(CN)6]3-|4- and

[Ru(NH3)6]2+|3+ as redox probe to study the electron transfer reactions on the

SAM modified electrodes. The charge transfer resistance (Rct), which is a

measure of the blocking ability of the monolayer, was determined from their

impedance values by equivalent circuit fitting procedure. In addition, EIS data

were used for the pinholes and defects analysis, from which the surface

coverage of the monolayer on Au surface was determined.

In the case of self assembled monolayer of 2-naphthalenethiol on Au

surface, the structure, adsorption kinetics and barrier properties of the

monolayer were studied using electrochemical techniques, grazing angle FTIR

spectroscopy and scanning tunneling microscopy (STM). The results of cyclic

voltammetric and impedance measurements using redox probes show that 2-

naphthalenethiol on Au forms a stable, reproducible but moderately blocking

monolayer. Annealing of the SAM modified surface at 720±20C remarkably

improves the blocking property of the monolayer of 2-naphthalenethiol on Au.

From the study of kinetics of SAM formation we find that the self-assembly

follows Langmuir adsorption isotherm. Our STM and FTIR results show that

313

the molecules are adsorbed with the naphthalene ring tilted from the surface

normal by forming a √3 X 3 R300 overlayer structure. From our studies we

conclude that the electron transfer reaction of ferro/ferri cyanide in the freshly

formed monolayer occurs predominantly through the pinholes and defects

present in the monolayer. However, in the case of thermally annealed specimen,

while the ferro/ferri cyanide reaction is almost completely blocked, the electron

transfer reaction of hexaammineruthenium(III) chloride is completely allowed.

It is proposed that the electron transfer reaction in the case of ruthenium redox

couple takes place by tunneling mechanism through the high electron density

aromatic naphthalene ring acting as a bridge between the monolayer-modified

electrode and ruthenium complex.

We have studied the structural integrity and barrier properties of self-

assembled monolayers (SAMs) of thiophenol (TP), o-aminothiophenol (o-ATP)

and p-aminothiophenol (p-ATP) on Au surface using electrochemical

techniques such as cyclic voltammetry (CV), electrochemical impedance

spectroscopy (EIS) and gold oxide stripping analysis. The results of cyclic

voltammetry and impedance spectroscopy measurements show that these

aromatic thiols form stable and reproducible but moderately blocking

monolayers. We have tried to improve the blocking ability of these monolayers

by potential cycling and mixed SAM formation with 1-Octanethiol and 1,6-

Hexanedithiol. From the results of cyclic voltammetry and impedance

spectroscopic studies, we have shown that the blocking ability of these

monolayers is significantly improved by potential cycling and by mixed SAM

formation, especially in the case of p-ATP on Au surface. It is found that the

electron transfer reaction of potassium ferrocyanide occurs mainly through the

pinholes and defects present in these SAMs. In contrast, the redox reaction of

ruthenium complex takes place through a tunneling mechanism mediated by a

314

π-electron bridge arising from the aromatic core of these molecules. From the

impedance data, a surface coverage value of 99.9% has been determined for

these SAMs on Au surface.

Similarly, we have also studied the self-assembled monolayers (SAMs)

of liquid crystalline thiol-terminated alkoxycyanobiphenyl molecules with

different alkyl chain lengths on Au surface for the first time using

electrochemical techniques such as cyclic voltammetry (CV) and

electrochemical impedance spectroscopy (EIS). It was found that for short

length alkyl chain thiol (C5) the electron transfer reaction of

hexaammineruthenium(III) chloride takes place through tunneling mechanism.

In contrast, redox reaction of potassium ferro/ferri cyanide is almost completely

blocked by the SAM modified Au surface. We also find that the rate constant

values of [Fe(CN)6]3-|4- redox couple are decreased by almost four orders of

magnitude for the SAM modified electrodes when compared to bare Au

electrode. From the impedance data, a surface coverage value of >99.9% was

calculated for all the thiol molecules.

315

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